Snivy never did enough damage, takes for ever to get anything good and just plain drags through the majority of the game.What dictates that Snivy's the worst? He is a great neutral attacker in my experience. Coil, Leaf Blade, and Return is all you need to fight a good majority of pokemon (especially in the E4). Granted, I also see Samurott as a very solid starter.
Emboar, on the other hand, is outclassed as a physical, special, or mixed attacker. He is too frail and slow to use his fighting typing well. Hammer Arm and Superpower are usually useful with a bulky or fast attacker. Emboar is neither. Brick Break is a PWT TM and focus blast doesn't come til later. His only advantages are Head smash and potentially heat crash.
My opinion is he is just too easy to substitute, unless you are desperate for a fire and fighting type on your team.
It is also outclassed by just about anything, especially if you're looking for a grass type. I'd argue he is more replaceable than Emboar just because of all the grass options to go up against
At least a move like Arm Thrust can be useful the entire game. (Emboar was marginally bulky in my experience with it in BW1 but maybe BW2 changed things up enough).