I remember a program once that used a model kind of like this. The main program itself was FOSS, and so were the Git and SVN plugins for it, but the ClearCase and SourceSafe plugins for it were proprietary and very expensive. (Unfortunately, I don't remember what the program was.)

This is one model of open core I can get behind: make everything FOSS except compatibility/integration with other non-FOSS things. It's a win-win: if you care about being 100% FOSS, you'd have no use for the proprietary plugin, and conversely, if you do need it, then you're already contaminated anyway.