It's not true that it will never reduce it. It wouldn't be too hard for a CPU to notice when the result of RDRAND is XORed with something, and do evils to deliberately control the eventual result.
fake_random = 5
result = secret ^ real_random
result = result ^ fake_random
current_random = <something unpredictable>
fake_random = RDRAND()
new_random = current_random XOR fake_random
The fact is that the Linux/BSD random number generators were actually rather good before all this RDRAND stuff.
fake_random = 5
fake_random = secret ^ real_random ^ 5
In a larger sense, there's no way to stop the processor from simply setting result to whatever it wants before returning it to the place where it's used.
My point is, there's no real way to trust hardware that can't be verified.