If each worker thread ties up other resources in an application, such as handles to server connections, conserving threads could have a significant impact. That's the situation for an application I am involved with.

I've written and tested a patch to make this change, using a second Queue for the worker threads to notify the executor in the main thread by sending a None when they finish a WorkItem and are therefore idle and ready for more work. It's a fairly simple patch. It does add a little more overhead to executing a job, inevitably. I can submit the patch if there's interest. Otherwise, perhaps the TODO comment in thread.py should be rewritten to explain why it's not worth doing.