(a)they'll feel a bit cheated by having to bring stuff to a wedding (once they find out), (b) the food may not be stellar and/or go together very well and give a shoddy impression, (c) people do like a good sit-down dinner at a wedding,
Soooo, it sounds like you have your food dilemma solved, but for those pondering the same questions, I have this input:
(a) I would never feel cheated about having to bring food to a friends wedding, I know I like the food I make, not so much with catered food I've had before. Besides, then I feel like I've been able to contribute to my friends celebration.
(b) if you are worried about the food going together, offer a theme to your invitees, so that the food they bring will kind of mesh
(c) Sit down dinners are restrictive. keep your guests moving, interacting, and having a great time with each other. It makes things fun.