Even if you're only paying them based on required qualifications rather than on their actual qualifications, you're paying them at below the market rate that they could command. If they ever decide they want to leave (for more money, or something annoyed them, or a friend of theirs is hiring) they can do so easily, and they're more likely to be tempted.

In this analogy, you can buy a Ferrari at the same price as a Miata but the Ferrari is much more likely to get stolen.