You don't need to publish a software as open source to allow people to write plugins at all.
As far as the definition of open source goes, nobody considers Windows to be open source just because some of Microsofts customers have access to the source code.
If it's worth saying, but not worth its own post, then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should start on Monday, and end on Sunday.
4. Unflag the two options "Notify me of new top level comments on this article" and "