If match making works....
If the match making system works, as people like to claim it does, why is there such a massive skill discrepancy in lower divisions? The highest I've ever been ranked was Silver 1, so I'm speaking from that perspective. A match-making system that allows players of such obvious skill differences to be at the same ELO/MMR is obviously flawed, and the fact that this happens is proof-positive of that.
When someone posts about being frustrated with bad teammates, people like to tell them they just aren't good enough to carry and, therefore, to advance divisions. My thought on this is, if the matchmaking system works, why would you have to carry your team on your back to advance? Should not everyone in your games be of a similar skill? How is it that there are such MASSIVE differences in skill levels of players at the same ELO/MMR? If the only way to advance is to carry your team, how did some of these players get to this ELO/MMR, because they clearly did not carry their team. The fact is, you shouldn't have to be able to carry your teammates single handed to advance. A player that can solo carry every game at bronze ELO/MMR belongs in Gold+, not in Silver, for example.
TLDR: there is too large of a skill discrepancy within the same divisions to be able to say matchmaking works properly.