I don't think I would have bought Black or White if there were no new Pokemon. New Pokemon is always a staple for new generations cos it helps make money.
Also, like Golurk said the new Pokemon are there to be used, it gives the games a "fresh look" however some people, although they didn't like being able to use their old favorites until the post-game, thought the new Pokemon looked silly, especially the starters. I actually grew onto the new Pokemon quite quickly, and that was thanks to me doing research on them on Bulbapedia. There weren't many Unova Pokemon I particularly hated, other than the Trubbish line.
If Unova had a Pokedex containing only old Pokemon, I would not have enjoyed it at all.