I feel as though everyone whose posted for both sides of the argument have had good points, but very few seem to grasp what the real issue is in comp. The issue is very simple. MMR should NOT be a factor at all in competitive mode. SR should be the sole factor in deciding teams. Balancing comp mode off of MMR is bad for everyone.
Imagine a platinum player getting unlucky in a string of games, let’s say they get game after game of leavers, throwers, feeders, etc on their team that results in a long losing streak, which is easy to imagine, since it happens to nearly everyone at some point. Now, imagine that plat player drops down to 1400 SR after two days of terrible luck. They’re legitimately a plat player, so their MMR still reflects that. What does this mean for how the matchmaker bakances games? Well, it’s easy. The game will take all those 1400 players, look at their MMR, and see this plat player who is better than the others. His MMR is high. To balance the overall team MMR, it places that plat level player with 5 below average MMR players to go against 6 above average MMR players. This equals out to a balanced overall MMR for each side. In theory, this makes the game balanced, but in practice, it turns into a stomp. That stomp will be against the plat level player’s team, unless he hard carries his 5 teammates, all of whom will be performing badly compared to the enemy team. If the high MMR player does not hard carry, he loses even more, rather than easily gaining rank back to plat, since he isn’t facing a normal bronze level team with normal bronze level teammates at his side. The game makes him hard carry in order to get a win. Thus repeats itself game after game after game, and it usually creates the huge grind and the 50-50 win/loss ratio everyone complains about.
Why is this bad? Well, first of all, a player who should easily climb in normal circumstances is now given a far more difficult path to get back to his/her baseline rank. They are forced to hard carry 5 potatoes over and over and over again to get back to plat, because they’re hindered by MMR being the mitigating factor in the comp matchmaking process. It’s unfair to them, all in the name of ‘balance.’ It’s unfair to his enemies, because the game forces them to play against a plat level player in a bronze level game. It’s unfair also because his teammates will be carried by him if he does manage to pull out a win. They essentially have an easier time to climb, even though they have lower skill, while the higher skilled player has a more difficult task.
If MMR was not used in competitive matchmaking, the plat level player would quickly win all his/her games in bronze/silver/gold, because they would be facing off against average bronze/silver/gold players, with the same calibur teammates. They would easily be able to tip the scales, and in effect, they’d bump back up to platinum very quickly. After that, they’d have a more difficult time ranking up, because they’d only be able to do so by improving to a diamond level player at that juncture. Luck or bad luck streaks would be quickly mitigated this way.
The issue is that the game doesn’t do this. Instead, it sees the high MMR of that player and faces them off against tougher competition, with lower skilled players on their side to balance the difference. In effect, it gives that player have a very tough grind to get back to the level they belong in. It’s simply unfair and against what a competitive mode should actually stand for. Why have SR at all if MMR defines how the game matchmakes?
The reason people still eventually rank up over a very long period of time is due to the fact that at bronze, there may not be as many players with the same level of skill as a plat player who deranked due to bad luck. Even though the game tips the scales against them, the matchmaker may not be able to find enough other plat level players who also deranked to place on the enemy team. Even though that high MMR player will be playing every single game against other above average MMR players for that bracket, they still may only be above average for their rank. It’s a large enough factor over time to tip the scales ever so slightly to where many games will be hard carried for wins, which otherwise were created to be more balanced with MMR averages.
What it does in every case is force a far tougher grind on the better players in each bracket to rank up to where they belong. It also gives lessor players an easier grind to rank up. It’s an unfair system that isn’t representative of what a pure competitive ladder should look like. Players stuck in lower brackets than they belong should naturally have an easier time to rank up, but blizzard decided against that in favor of balanced matches over short term volatility. While games would be stomps early on in tge season without mmr balancing, they would naturally level out as each player got to the bracket they truly deserved. Blizzard decided that the short term volatility would be a bigger negative on player perception than the unfairness that MMR balancing creates in the long term. The big issue is that the majority of players have seen how the system punishes good play, so the overall backlash is just as great, if not more so, than what would have been with a system that balanced games solely on SR.
So maybe I did not express myself correctly. 
