Just a note I'd like to make on Toujours' post, branched evolutions that can only be obtained in Unova is a terrible idea. It would mean that without trading from a Generation V game that is not in Unova, we wouldn't be able to 'Catch 'em all'.
As for as to if they should have made new evolutions in B/W for past generation Pokemon... I'd say no. Primarily because of what Reshiram Man stated; B/W were meant to be the beginning of a new journey, comparable to the original games on the Game Boy in scale. Had they stuffed in new evolutions of old Pokemon, it would have felt forced, regardless of how much I want to see Eevee's evolutions come full circle.
The point of B/W was to branch out the lands of Pokemon in a way that made it feel like a whole new adventure, with all new Pokemon wherever you turn (until you complete the game, of course!) and all new people for you to meet... and I feel they've achieved that. It may not be as grand or well-received by old fans as R/B/Y were, but it comes across as a new and exciting adventure, and for that is part of why I like it.