Honestly, it is kind of pointless. I've seen mastery 7 people who are terrible. I understand what this thread is talking about. I had a dude first time Nunu support or something in ranked once because he thought he'd seen it be good or whatever. I don't know. It was terrible.
But the thing is, master restrictions don't really reliable help that. Yes, someone with mastery gives a higher chance of familiarity with the champion. In my opinion, though, that skill level is debatable regardless. I've seen mastery 7s who deserve their mastery, I've seen mastery 7s who were not... very good players in general or at the champion, but have played a lot, and somehow hit the tokens, and I've seen mastery 4 players who put the 7s to shame.
Mastery gating as a way to better ensure people can play their champions doesn't seem very reliable to me. Sure, they may have basics down, which is better than 1st timers, but I don't think that will reliably improve your chance of victory as much as you may think.