They make your pokemon fat and diabetic. Good luck finding insulin at the pokemart, and they won't let you take it out of the pokemon centers, either. --- I'm sick of people that suck at video games blaming games and developers. Listen, it's not their fault you're lazy, dumb, and have the reflexes of a dead cow.
It really only matters in competitive battles. Rare Candies prevent you from gaining EV's as you level up, making your pokemon weaker overall. However, they're pretty useful in-game because you gain EV's throughout the game. Besides, three levels would make nearly no difference. So go ahead and use them, you have nothing to lose.
From: VGPod3 Umm... What are EV's, and how do I tell how many I have, or when I am "done"? *shrug
Don't worry about it. You can use 1,2, or even 3 rare candies and it won't affect your pokemon. --- I do have an idea, but let's run some expensive tests just to be sure http://i153.photobucket.com/albums/s231/Wiiman741/4th.png
Effort Value at it's simplest is just an invisible number for each stat on a pokemon that raises based on the pokemon you beat. You gain them at the same time as experience points. It was a system implemented to make "trained" pokemon seem stronger than ones that were just caught at a high level, or rare candied to level 100 (wish I knew about EVs in Red and Blue >.>) --- >:I Been off the presig cold-turkey since: 23/7/07