> if you know one source is bad, might as well take it out.

Yes and no. Mostly no.

In a simplified model, it's only useless if it adds zero bits of entropy. But if a source that's supposed to add 128 bits of entropy only adds 16, well, it's still 16.

I would never trust RDRAND on its own. If nothing else because it's always subject to a microcode backdoor. But if I already have something I'm happy with the entropy of, sure, I'd XOR it with RDRAND output. It cannot make it worse.