It the case of an artist, time is money, so while they work on the request they've taken up that gets cancelled, they could have been working on another request instead.
Obviously there'll have to be some time limits and a few exploits to find solutions to, but I think the concept itself is good.
It would require a slight rework of the request system though, at current any number of artists can start work on a request, and they dont have to tell the customer that they did, they can just present a finished product for sale. The suggestion wouldn't work with this kind of system, so a more formalized request system would have to be established.