Pros: Bantams eat less (less poop!), need less coop space (you can fit more in!), yet still produce a decent size egg. They're easier to hold/handle if you want them as pets, or if you've got small kids. Roosters are much smaller in comparison to standard roos, and some (my cochins) have quiet, pleasant crows that are easy on the neighbors.
Cons: If you sell their eggs, you'll have to explain about the size, and maybe people would prefer to buy larger eggs. If you're ordering bantam chicks from hatcheries, they don't sex bantams, so you have to order straight run. Some roos (my japanese) have high pitched, screechy, (some would say) obnoxious crows.
I'm pretty sure bantams convert feed to egg in a more efficient ratio than heavy breeds, who would eat more and make less egg in relation. Someone can correct me if I'm wrong here.