Non-Exclusive:
Well, it's cheaper.
Oh, and surfers might be inclined to buy from a gallery simply because they're tired of seeing the same stuff over and over.
Exclusive:
Easier TGP listings.
More relaxed license terms. Can usually be used on hosted galleries, as free content, etc.
More sales are possible because surfers haven't seen it before and it'll keep them on the page longer so they see your ads longer.
Members like fresh, unseen stuff.
You can get shoots exactly like you want them. Sometimes it's hard to find what you're looking for in non-exclusive. (Like if you need a specific type of model in a specific type of clothing in a specific setting.)