I don't really care if someone makes an order and then doesn't pay - it doesn't cost me anything, and the order is automatically cancelled by Bitmit after a few days.
It means you are granting them a free put option on their bitcoin. If you are selling money at something close to fair value then this can cost you something.
Well, both parties can rate each other, so yeah, no big deal. Just down-rate if you feel like somebody is abusing your fair prices. Same problem exists on localbitcoins where people blame others to only do deals if the price moved in their favor.