Would be neat. From what I understand, doing the Pokedex only gets you a diploma in any games besides the Kanto ones, and I don't think it's a fair reward for all that time spent. Honestly, I don't really think it's a reward at all. It's just a piece of paper that you can't really do anything with.
As long as it wouldn't be something required to progress in the game and would be something extra but really useful, though. This is probably my third time saying this, but I'm personally not interested in catching them just for the sake of catching. It's just tedious for me. I actually went through Rock Tunnel in FRLG in near-darkness because I dreaded even the 10 entries requirement for Flash. I think collecting should strictly be a personal thing, like Contests and battle facilities, that you can do if you want but don't have to.