It's actually a good idea, but its horribly hard to implement and it won't last for long...

one can't force people into stating his reasons, I for one, would actually quote the first person to vote for the one i want.

or i just could state my reason as "just because i like it" or "because its made by (insert username here)"

some people don't have the expertise to suggest what to fix on a sig or what not, so you can't force suggestions neither.

I repeat, Nice idea, but it would be almost impossible to properly implement.