Aggregating multiple sources of randomness has the potential to conceal bugs. In the Debian/OpenSSL bug from 2008 [1], randomness was sourced from multiple locations including the current process id. The idea was that more randomness, even the minimal amount from the pid, could only increase the total entropy. However, when the primary source of randomness was eliminated through an overzealous patch, the PRNG still emitted plausible looking numbers due to the remaining sources of low quality entropy. Had the PRNG only used one high quality source of randomness, people would've noticed something strange about their generated private keys much sooner.