I had 2 Mewtwo's in LeafGreen, and the one in Diamond is from my old game, because I traded about 10 of my best old LG pokemon to Colosseum and then started a new game and then I beat the game, caught legendaries and traded em back. So I have like Mewtwo and others (old LG and Colosseum), oh and my Espeon (and Umbreon who i didnt use) are from Colosseum =D.
Umm....you can raise legendaries. And it's a lot harder to train pokemon at that level than level 30s. It's 100% fair because you guys have the exact same chance to get them too.