While that's true, just blindly mixing all you've got is a bad idea. Let's say you have "randomness" sources A, B, C and D, and you just use A+B+C+D. Now an attacker has total control over D. Then she could adaptively feed you with a D value that is canceling out A, B and C. Result is deterministic output.
Just adding in more is not always good. You need to trust every source. Or more precisely: You need at least one source in the mix that an attacker can't predict/read out.
Just adding in more is not always good. You need to trust every source. Or more precisely: You need at least one source in the mix that an attacker can't predict/read out.