It'd be demoralizing
Yes, that too. Currently, if your design brief doesn't pass, it encourages you to go back to work and figure out what's wrong with your design concept, then submit it again. If we had "rejected" stamps, it would be like a blow to the mapmaker's face: "this is crap, you shouldn't waste your time on this any further" and perhaps projects that could have become something great will be abandoned...
I like the current system actually, the design brief approved -stamp is good idea, it's like an encouragement to the mapmaker to keep working on his project. A "rejected" stamp would have the opposite effect, obviously.