Train as in In-Game or for battles against friends?
Weeell, I like to go trough games with Pokémon that aren't that great (My current team in Emerald has only Fire types, and I plan on keeping some of them as they are, without evolving them. Hmm, I guess I will have some problems in PokéLeague

') because sweeping trough the game with powerful Pokémon... just isn't fun. :/ I'm supposed to beat Emerald with teams always having only one type, until I've used all types. Of course, I won't use legendaries in those teams, and use my favorites, not the strongest of the type. And after I've used all types? Hmm, I need to figure out something... Maybe I'll mess around with a randomizer...
However, in my Ruby, I train whatever I feel like training. Though they are mostly at least last forms of Pokés. And yes, I do train Ubers in them. I always love training a Ho-Oh. Now that I'm talking about it... I feel like sweeping the dust from my FR and training myself two more Level 100 Ho-Oh <3
Against friends, well, I use what I've trained, I don't exactly train just in the purpose of using it against someone.