When exactly is your opinion allowed to count? As a 4k player you can sit here and say that a 2k has no business contradicting your opinions, but then a 5k can say the same to you, and a top 100 can say the same to them. Then professional players can enter the discussion, call all of you shit, and then say your discussions don't even matter because pro games are the only place where the real game is played. An open discussion means anyone can contribute, and if people are giving bad advice you can quote them and correct it. Sure, they won't ever admit to being wrong but if your arguments are decently explained and make sense you should get through to the third parties reading. Attaching mmrs to posts would make people more toxic and unwilling to put forth their ideas, I can imagine many people in this thread not even wanting to post if that was the case.
And while mmr is a decent general indicator of your skill, it isn't an insanely accurate end all. Solo mmr measures how many games your willing to play alone and how good you are at winning them alone. This favors playing snowballing mids extremely selfishly, or spam picking your best snowballer/fotm over and over. Say what you want, but the best 5 position player ever is going to have an extremely slow (and unpleasant) time climbing compared to sniper/troll/sf/meepo/whatever spammers.