Some games do not have a published maximum. It wouldn't work in those cases.
True, but most do. I'd guess it would work for 95% of the games in the database. Alternatively, just make every poll run from 1 to 7+ - that would be sufficiently detailed for 99% of the games.
Polls seem so "impersonal". I'd much rather read the discussion around a similar topic than to see just a meaningless number.
I agree that there are subtleties about optimal player number that are better served by discussion. But the same could be said about the overall game rating just as easily. As a quick and dirty indicator, I think it would be very helpful.
Seems like a great idea to me.