I don't REALLY like to level up my pokemon in the day care because I prefer to raise them in my own personal...conditions. (ergh, sorry, didn't word it right. ;; CURSE MY ENGLISH SKILLS.)
I mean, fighting wild pokemon that gives the right EVs to my pokemon, that's all. Because when I play the game, I kind of prefer to also "EV train" here and there. Plus, the majority of pokemon I raise evolves from happiness as well, so...yeah.
Plus, the lucky egg comes in very handy in rasing in Pokemon Black and White; MUCH more eaiser and quicker IMO.