Rebuttal towards preference of Scizor
I'm currently going through the process of putting a team together involving Scizor, and I happened to take a look at Scyther's page on Serebii while I was at it. I was slightly surprised to find that the two had an equal amount of total base stats. In fact, looking at it, it seemed to me that if anything Scyther had a better distribution than his evolution, losing some attack and defense in return for a speed stat that outruns a good portion of pokemon. So which one is more viable to use?
The biggest problem with Scyther is that it has a middling speed which can be easily beaten by common sweepers in the metagame right now, and it can't really come in on anything except perhaps a revenge kill. At 105 base
http://www.smogon.com/forums/showthread.php?t=23432
It registers as a blip on the radar, requiring it to run Jolly 252 to maintain a competitive speed at its rank. Infernape (Naive/Jolly/Hasty) will always outrun with standard builds, not to mention Gengar/Azelf/Starmie/Raikou. All of which can come in to revenge kill/set up.
"Some attack and defense" Equates to (when all things are neutral (aka Hardy nature)) 40 attack points and 40 defense points at lvl 100. That being said, Scyther is easily walled by most rock and steel types while the inclusion of STAB Iron Head allows Scizor to forgo the usage of a lackluster non-technician non-stab Brick Break in lieu of a Stat boosting ability (Agility/Swords Dance come to mind). The big threats that Brick Break is meant to deal with are handled easily by X-Scissor/Iron Head/U-turn anyway. (Tyranitar/Weavile come to mind). Steels that are weak to Brick Break are usually things that Scyther has no business staying in on, (Heatran and Magnezone come to mind, Heatran outspeeding with a Choice Scarf, Magnezone easily surviving non-stab non-choice band Brick Break and hitting back with a fatal thunderbolt)
The obvious advantage Scizor gains is his typing - Steel/Bug suffers only one uncommon weakness (albeit 4X) in fire, and has resistances to normal, grass (4X, making him great with a Swampert), Ice, Psychic, Bug, Ghost, Dragon, Dark, and a poison immunity to boot. Bug/Flying, on the other hand, is notorious for being one of the worst combinations in the game, packing weaknesses to fire, electric, ice, flying, and worst of all, 4X rock. However, it's resistances, while more limited, are stronger in some regards than Scizor's; 4X grass, 4X fighting, and complete immunity to ground (and a bug resist :p).
Uh, how is Scyther's 4x Grass resistance with less defense "stronger in some regards" than Scizor's 4x Grass resistance. Same with the 2x Resist to Bug. This quote should actually read:
The obvious advantage Scizor gains is his typing - Steel/Bug suffers only one uncommon weakness (albeit 4X) in Fire (Scyther shares a 2x weakness to fire and with its defenses cannot take any metagame standard fire move (Flare Blitz, Overheat come to mind) anyway using a 252/252 att/speed build), and has resistances to Normal, Grass 4X, Ice (in an environment swarming with Garchomp/Salamence counters), Psychic, Bug, Ghost, Dragon, Dark (covering the new Physical Crunch and addition of Night Slash), and a Poison immunity to boot. Bug/Flying, on the other hand, is notorious for being one of the worst combinations in the game, packing weaknesses to Fire, Electric, Ice, Flying, and worst of all, 4X Rock. However, it's resistances, while far more limited, are stronger, in rare instances, than Scizor's; 4X Grass (Which Scizor shares), 4X fighting (Who have commonly carried Rockslide and now Stone Edge), and complete immunity to ground (and a bug resist :p).
It also doesn't help that Scyther takes 50% from Stealth Rocks which basically what teams must be built to handle or dole out nowadays. (Common Stealth Rockers being Hippowdon and Bronzong, or even Blissey). But the fact that you must use it as a starter to ensure that it doesn't switch-in to 50% damage also screams "Switch-in a physical wall that can set up Stealth Rock on me to ensure my limited usage!" It's a double-edged sword. If you don't choose to Choice Band you deal middling damage on non-STAB attacks and are easily Walled/Set up on by Pokemon that simply recover off the damage or destroy you with priority moves (Dusknoir laughs at Pursuit and Will o' Wisps to destroy any chance of sweeping, Donphan/Mamoswine are fairly happy to CB-Ice Shard you)
So why use Scyther over Scizor? Speed makes more of a difference than one would think. While Scizor is likely forced to absorb two hits (unless he is Scarfed) before he gets an attack when he switches in, Scyther will usually get the jump on his opponent the turn after he switches in, meaning he can actually serve as a decent counter to pokemon that are middle-of-the-road in terms of speed. Not only that, but by switching in on a predicted fighting or ground move (very common attack types), Scyther takes notably less damage than a switching Scizor (who would take normal damage to those attacks).
D/P is, in all actuality, a fairly Bi-polar metagame, while one spectrum strives to reach faster and faster speeds via nature and Choice scarf (try to find one RMT without a Timid/Jolly/Naive Pokemon, I dare you) the other spectrum is running Impish/Bold/Relaxed 252/252 HP Defense like it's going out of style, Trick Room teams are everywhere, and relying on speedsters that don't bust Bronzong/Dusknoir (and by proxy threats such as Rhyperior or Rampardos) wide open can be a liability to anyone.
another big issue is switching in: Sure, Scarf-less Scizor will take 2 hits before it can attack if the target stays in, but I would put money down that it has the resources to survive those two hits, especially with a bulky 200+ HP EV investment. Scyther, on the other hand, falters to a 2 shot mentality on a neutral damage switch-in, most particularly in regards to the types that Scizor conveniently resists: Normal, Dragon, Ghost, Dark, Psychic. With Choice Specs and Nasty Plot introduced in 4th Generation, there are far harder hits in regards to these catagories, most particularly Tri-Attack, Draco Meteor, and the newly special Shadow Ball.
The biggest problem about "switching in on an Earthquake/Close Combat" is that the opponent is either ready/willing to take a shot at you with Stone Edge the next turn, or will just switch out. And since switching out is priority +7, it really doesn't matter the speed of the Scyther, without Pursuit it won't catch its fleeing quarry. Scizor on the other hand, may take advantage of the situation and judging with its resistances, can make easy determinations whether the opponent will decide to switch out, thus evening out the "speed" playing field.
If the opponent cannot attack Scyther with at least a neutral hit (a rarity given the plethora of moves pokemon with Close Combat/Earthquake fall back on for additional coverage), or knows for sure that Scyther (and its fairly limited move type range Bug/Flying/Dark/Fighting) will threaten and outspeed it for a potential KO it WILL flee. There is almost no question about that. The main advantage given to Scyther is that it can use this rare window as an opportunity to Stat Up. The main disadvantage is that, even statted up, it will not do fatal damage to the slow and likely 252/252 built physical walls that it faces.
I believe you're stuck on the "plain fact" that
with his speed Scyther can make better use of a CB set than his metallic counterpart (again, two hits versus one)
When, considering you've turned the blind eye on the fact that Scizor has far more survivability without having to "invest" 252 Speed EVs just to remain competitive. It still outspeeds the majority of tanks in this Gen, who seem to love being base 50 and below, and with the option to run Jolly 252 Scarf, Scizor hits an enviable 379 Attack, 376 Speed. Safely outspeeding Timid 252 Azelf and Starmie, Timid 252 Alakazam and anything else below the 120 Base Speed tier.
EDITTED IN:
It is also crucial to add that Scizor with its laundry list of resistances and 20 more base Defense makes for a far better user of Roost than its preevolution Scyther, carrying with an Impish 252HP/252DEF/6Attack build a very respectable 344 HP/328 Defense/297 Attack, An Iron-Red Bug Tank if I'd ever seen one. Swap some numbers and natures around and you can get an Adamant 252 HP/252 Att/6 Def build, running 344 HP/237 Defense/394 Attack, more of a heavy Artillery that can take a hit and fire back with awesome force.
Wow, that was far longer than I thought it would be. But it's basically an analysis on the overall feel of D/P. Resist or be killed, outspeed or bulk out, run Scyther with rapid spin support and something that can kill ghosts or run Scizor with Heatran and demolish with Flash Fire procs.