Part of me agrees but overall I think I don't agree.
People can post clues without stating the clue's source Alternatively, they could lie about the source. In both cases, how would a potential 'answerer' know not to provide an answer?
Anyway, even with an embargo on this site, answers might just become more readily available on A
I don't know how many people send in correct solutions for these prize crosswords. Let's assume 20,000 per week for the Saturday Times prize crossword. The additional 20 people (say) who send in a complete solution solely due to answers posted on this site - and who would otherwise not have sent in a complete solution - don't significantly alter the miserable odds that you currently have of winning the prize.