Earning money

I think that, in order to have an "open core", you really have to have a "core" as an entity. If you do, it's perfectly reasonable to have proprietary plugins, and there aren't the same conflicts over whether you'd accept contributions that would compete with them, because it would be a layering violation to accept plugins in the core. Open source plugins, whether or not they compete with proprietary offerings, are a separate project from either the core or the proprietary plugins, in much the same way that mod_python is separate from apache.

The issue is that a lot of companies seem to be claiming an open core when they don't have a division between a core and non-core in the first place, but rather have a single project with more and less functionality in branches. This puts the maintainer in a conflict of interest, rather than cooperating on one front while competing on a different front. It only makes sense to make something open source when you and the community would both be motivated to make the part you share as complete as possible within the scope of that part.