MMR and Division/Tier placement should be the same thing. Not separate entities.

If my current division is Gold 5, I: should have Gold 5's on my team and Gold 5's as my enemies. shouldn't have anything else anywhere.

If my current division is Plat 2, I: should have Plat 2's on my team and Plat 2's as my enemies. shouldn't have anything else anywhere.

If my opponents are Silver 4, I: should be in Silver 4 and my teammates should be in Silver 4. shouldn't be a skilled Silver 5 or an unskilled Silver 3. shouldn't be Bronze 1 with Silver 2 teammates or Silver 2 with Bronze 1 teammates.

If my opponents are in Diamond 3, I: should be in Diamond 3 and my teammates should be in Diamond 3. shouldn't be a skilled Diamond 5 or an unskilled Diamond 1. shouldn't be a Platinum 2 player with Diamond 1 teammates or a Diamond 1 with Platinum 2 teammates.

My MMR and my placement on the ladder should be identical. Not separate. This whole system is stupid. Promotion matches are stupid. The current system has no balance in it whatsoever. It's trying to weigh your skill + your allies skills + your enemies skills and create a "balanced" match, but what it fails to do is actually calculate your opponents.

The system looks at your team and says, "Well this team has three Gold 4's, two Silver 1's and one Silver 2. So the enemy team will also have three Gold 4's, two Silver 1's and one Silver 2."

What the system CAN'T see is that the enemy Silver 2 is playing support while the Silver 2 on your team is playing mid lane APC against one of the Gold 4's. The system is failing to balance by trying to balance. The whole game should have one rank in it at the most.

Promotion matches SHOULD WORK by you being able to CONSISTENTLY beat opponents at your level. If you're Silver 3 and you CONSISTENTLY beat other Silver 3's, you are moved up to Silver 2. If you CONSISTENTLY lose to Silver 2's, you drop back down to Silver 3. Not with fucked up MMR's and a system that has no balance whatsoever.

TL;DR: Current system sucks and works on too many approximates. Make it more accurate.