It doesn't matter that much, does it..?
It's an interesting concept but I'm not sure plugins will behave cleanly with it, I suspect unless it's managed pretty much automatically they won't use it.
They don't have to. They simply can...
Mind you, most plugins will only use PURLs features that are managed automatically (very few plugins for SMF ever cared, and those that did, were pretty much portals and/or PURLs type plugins in the first place), so if it can be done transparently and 'just work', plugins can use it but otherwise they probably won't.