GPL+Commercial requires copyright assignment, which is a pretty sure way to turn off potential contributors. I mean, if you don't want to contribute to BSD-style licensed projects because you don't want others to leech without giving anything back, how on earth could you simultaneously be ok with assigning your copyright to some (sleezy/evil/you name it) corporation?

And no, I don't really have an answer either. Successful open source business models are few and far between. Redhat is perhaps the one big example of a (very) successful open source company.

To some extent I think the worry about leeching is something which can be addressed with culture as well as licenses. Plenty of successful BSD/apache licensed projects around, including with largely corporate contributors, because they understand the long-term value from contributing to the common pool rather than forking and trying to sell "proprietary enhanced" versions.