Bad idea, collecting spell ideas is something you should rather do in a forum thread with some name like: "spell ideas thread", which would be a useless thread anyway. If you want spell ideas, then why not ask for them in a thread instead: "Making spells! Spell ideas welcome!" Or if you need help on making a spell, for example a bounce spell, you can check the forums or the spell tab, there is plenty there, or just make a thread called: "Help! Need Bounce Engine! Links and ideas welcome"!
So my answear is: No, a spell idea contest is completely useless.