s/slightly/completely/ (I believe my description of the core problem was right, but my description of why that problem exists was wrong - it actually has to do with the way mod_wsgi handles virtual hosts and endpoints)

If we expose an official way to switch the destination of the PyGILState APIs, then what it means is that mod_wsgi can, over the lifecycle of a single process in the pool, *switch* the virtual host and WSGI endpoint that process is handling by changing the active interpreter. There are still some extension modules where that won't work (because they create persistent threads that periodically call back into Python, so they may still end up calling back in to the wrong interpreter), but it will allow those that just do callbacks from external worker threads (or other operations that are similarly bound by the lifecycle of a request) to start working properly.