Good point, but ExpertsExchange has ads too. It also behaves in a spammy way, burying the answers at the very end of the page. That should be detectable behavior.
I bet there's a lot of things that spam pages have in common with regards to things like page placement.
So, if Google gets better at quantifying "spammy-ness" there will be an initial drop in spam effectiveness, but then spammers would catch on and emulate the behavior of good pages.
Eventually, the spammers' behavior would be indistinguishable from genuine posters.
