In CC there used to be a colossal reward of 1$ paid by some motivated member each month.
Perhaps the reward of the challenge could be simply to have a game made (pretty much like Ludum Dare's line, with mainly exposure on Scirra's forums).
I agree that prototypes and offering support in the forums allows to work one skills and keep things interesting/fast paced.
But "gaining" the experience of completing a game is also good and has deep impact on the whole process of game making, from the prototype to the release phase.
(Also it legitimises your position as game maker)
Actually completing a game apparently can be considered one tough task to handle in game making by many people (according to the results of this poll).
This site can be considered as a mainstream source.
So yes, having some challenges allowing members of our community to complete games is a nice idea in my opinion ^^