I know that people feel so-so about this. Some love to mix and match. I am one of them. Others love to keep their deck clean and only build with one generation.
One way people do this is by running through a generation, building a deck and then testing it. If they can't balance the deck they will go and mix in other cards. This works. But it is still playing on the idea of staying with a single generation as the base. This can lead to weaknesses that that generation brings with it.
For me this does not apply because I go by theme. I always do fire/water/grass (or electric) and often dark/fighting/psychic type trios, too. Like this I can use whatever Pokemon I like in the position that I feel is the best. This goes back to my rogue deck style of play, but it is also a strong strategy. I prefer having a mix of new stuff and old favorites on my teams, although I do also enjoy familiarizing myself with new Pokémon, and I do enjoy team building with limitations in mind.
Sometimes you just want to have a team tat plays strong against a certain other team. After all the game is about the experience.