I think some sort of money back guarantee can't hurt. The problem is figuring out how something like this could be implemented.
At least if you pay with Paypal they now have a 6 month buyer protection guarantee, so if you don't receive your item, or if the project deems fraudulent, simply file a dispute.
I think that as a backer, I'd be more confident. I suppose it depends on the amount of the pledges; if I lose 5-10 bucks, it's annoying, but not much else. Spending $250 on a product that never materializes, on the other hand...