Lots of people hire a graphic designer with no real idea of what they want--the point of a graphic designer is to talk to a client about the ideas and values of their business, think a lot about those things, go through a bunch of iterations, and then eventually end up with a finished product.

That takes a lot of work--usually at least 30 to 40 hours for a good logo. Would you put in 40 hours of work for somebody on the off chance they might pay you?

You can rest assured of this as well: people will definitely submit to your contest--but none of them will be great designers, because all of them will be people who place the value of their work at somewhere around "maybe 500 dollars."