VanQQisH said:
No one has mentioned that a Pokemon requires less experience to level up in its unevolved form. So a level 50 charmander will require less experience to hit 51 than a level 50 charizard would.
That isn't exactly true. Pokemon in the same evolution tree will require the same amount of experience to level up, e.g., a level 18 Bulbasaur might only need 800 EXP to level up, and an Ivysaur at the same level and with the same amount of gained experience as the Bulbasaur will also only ned 800 EXP to level.
The only reason to cancel evolutions is the movesets--when the moves are learned and what moves are learned. Examples:
-Kirlia learns Psychic at level 36. Gardevoir learns Psychic at level 40. (B/W)
-Trapinch learns Fissure around the Lv. 80 mark. Vibrava and Flygon don't.
-Smoochum learns Psychic at level 38; Jynx never does. Jynk learns Lovely Kiss at level 8, 22 levels before Smoochum evolves (and Smoochum can't learn Lovely Kiss). There movesets are also different enough to make a difference: Smoochum is the only way for Jynx to get good special Psychic attacks naturally, and Jynx learns many more physical attacks than Smoochum does.
EDIT: Also, Pokemon that evolve with a stone usually learn all their moves before evolution (e.g., Victreebel, Arcanine, Starmie learn less two moves after evolution). The only exception I can think of right now are the Eeveelutions (Vaporeon, Jolteon, Flareon), for reasons that should be obvious.