I've always looked at charging for open source as a support service. What is the developer's time worth? If one wants new features pay a fee/salary/bounty for the development.
Completely open source software, but you can pay a reasonable amount for managed hosting and support. Plus obviously it supports the devs and new development.
A simple line to separate DIYers from anyone who wants the convenience, peace of mind, or to support the project itself. Would love to see more of this.
In other words, I don't think it's a sustainable way to support open source projects