Open source plugins cause a lot less grief. Typically they don't have a feature I want. Often for legal reasons and not technical, or because a proprietary vendor is fighting back (eg; video patents the first case, Skype protocol the second).
The linux kernel is an example how an open-source-only plugin system works technical wonders. A very interesting case study was graphics drivers circa 2005. The closed source drivers (essentially plugins) tethered the linux community to the technically obsolete X server; and would have crippled the kernel in a similar way if the kernel devs had accepted closed source plugins.
Apple did wonders driving open standards on the web with the explicit acknowledgement that popular closed source plugins were too dangerous for their platform to implement .
The issue here is the one Stallman has been harping on since the dawn of time - closed source is unmaintainable in an extremely profound way.