I don't really understand why I've seen a few people saying that MR 4-5 should be required to play Ranked - I can kinda see MR 6-7, but even then, that's got a lot of downsides and its fair share of technical requirements to mandate.
But, backing up, here's the biggest problem with wanting MR 4-5 as a requirement for Ranked play; Literally /everyone/ can get MR 4-5 on a champion with enough time. They could completely suck as a Champion, but still hit MR5 with enough games they piggyback on their team with. Granted, that many games would still give them plenty of time to become familiar with a Champion and their abilities, but that's not really a great thing on its own.
That aside, I don't see any really good reasons to try and raise the bar of entry to fit with the Mastery System.