Yes, this is the biggest reason why this is required.
This, and and os.closerange().
I have browsed the CRT source and I find no way to determine if a fd is actually valid, before calling any of the functions that check for it.
Reintroducing the assertion will prevent a debug build python from passing the testsuite.
Much as I think this is useful (turning on the crt tests helped me find a couple of python bugs recently) it unfortunately needs to be off because of os.close().  All other troublesome cases (strftime(), open()) have workarounds.

Kristján