We've never said anything as such.So we should never use 252 HP on a Defensive Pokemon?
I stated that 252 HP may not always be the best idea on some Pokemon as it ends up being worse than it it was one point lower. I'm talking about when you get no gain in Leftovers recovery, yet increased Stealth Rock/burn/seed/whatever damage.
For example
344 (base 70's max HP) /8 = 43
344 / 16 = 21 (rounded down)
(21 * 2) - 43 = -1
Now that's a pointless waste of EVs no?
That and if you should not be using a Leftovers number and is not holding Leftovers (or get Lefties cancelled out by weather).
Example:
384 HP (Max) Zapdos in a sandstorm
Leftovers get cancelled by Sandstorm
384 / 16 = 24
384 / 8 = 48
384 / 4 = 96
So in a case where your opponent manages to predict your Zappy switch in three times and forces it out each time while Stealth Rock and Sandstorm are active, you'd have a dead Zapdos the next time you swap it in. However, going one point lower in HP still maintains that same defensive ability, and at the same time Zapdos wouldn't die the fourth time it switches in. Yes, Sandstorm will kill Zappy, but that's at the end of the turn, and you could take a hit from the opponent on that turn and die as a suicide bait. It's better than having the opponent hit no target, because negative side effects happen when they actually hit a Pokemon (whether it may be LO recoil, Draco Meteor, Close Combat, or even Pressure in extremely rare situations can hurt the opponent a bit). And also there's a chance that you can bring Zapdos in safely after your opponent kills something on your team, and Roost off the damage.
tl; dr: 383 HP > 384 HP on Zapdos in Sandstorm
Actually let me restate something: Don't go for Leftovers numbers in general (looks at Vaporeon analysis), because their benefits are often canceled and there are more things that indirectly hurt you.
These are simple things that are easily overlooked that give a very slight benefit (or more depending on your situation, such as bringing CB Scizor on an HP Fireless Celebi's Leech Seed) at virtually no loss.