This is already a problem in current ranked games when you have an 800 mmr and a 1200 mmr premade they are 1000 combined and would be matched with and agaisnt 1200 mmr players, but here is the catch. that one 800 mmr player is not on the same skill level, often resulting in too much feeding bad warding bad teamfighting, your team is now literally 4,5 players and the enemy team is not. So 3 other teammembers suffer because 2 ppl wanted to play together.
In normals i couldn't care less but in ranked this is annoying as hell. Sure this doesnt alwasy happen sometimes they are both 1000 mmr and its less problematic but they are still worse then the rest fo the team.
And sometimes you dont even notice they are there that happens too.
But i dont think non premades should be punished for matching premades against higer tier opponents they quite often can not handle yet. Its not healty for ranked games.