I can't speak for everyone but I for one really like the Pokemon that have been showcased thus far. Enough to where I would potentially use them on my team. When Blitzle and Woobat were shown in the CoroCoro leaks years ago I loved them and what do you know, they ended up on my team.
A better question is "why not". If you see a Pokemon that you really really really like the look of, then of course you're going to give it a tentative spot when you get the game.
I myself will definitely be using Talonflame because it is based upon my favorite animal. There will be no ifs, ands, or buts about it.
By that logic there's no reason to use any Pokemon and any point in the game as there might be something better revealed when you progress further. We're not going to get very many more before the release, leaving the rest to be revealed when we play it.
You're not forced to just use six for the whole game.
I think I like speculating about my team so much because I really like a lot of the Pokémon that have been announced so far, so it's fun to make plans. For DP and BW, I wasn't really too thrilled with a lot of the early-released Pokémon, but I love pretty much all of the ones they've shown so far for XY.
