Right, but the issue is that 99 out of 100 designers will do work for you and not get paid for it, while one will. There are some good links to thoughts about speculative work here: http://www.nospec.com/articles
The other thing that will happen is that the most credible submissions will come from places like Indonesia and Vietnam and Bulgaria. I wonder what freaks pro designers out more, concern over the wasted time of contest losers, or concern over the extent to which offshore piecework designers can drive the prices down for people like Colin who simply don't want the full 40-80 hour project.