Of course, when I have 300 connections to remote nodes, I use poll()
to multiplex between them.

But there are times when you can have a large number of threads
running concurrently, and if many of them call e.g.
subprocess.check_output() at the same time (which does call
subprocess.communicate() behind the scene, and thus calls
select/poll), then one extra FD per instance could be an issue.
For example, in http://bugs.python.org/issue18756, os.urandom() would
start failing when multiple threads called it at the same time.