I Just want to bring up one of my own posts from some weeks ago, talking exactly about the PBMM.
And to answer to your post: Yes, maybe the PBMM system didn’t work properly on GrandMaster levels, since there it takes really a lot more than having good numbers in a game to actally be good, but for lower leagues it could have made a big difference. Up to gold and probably even diamond it’s quite easy to tell a “good” player from a “bad” one by stats alone - a tank that always dives into the enemy team and dies the whole time, since he used his escape to go in; a healer that is sololaning with a lot of xp contribution but barly any heal; a specialist with a lot of siegedmg but no hero dmg, since he never came to any objectives; an assassin with less dmg than a tank, since he never hit any skillshots; and 1st of all - a player with lots of death, because ppl don’t understand that dieing is the worst thing in the game to do, even if you are a Leoric, Tyrael or Uther, that has some “trait value”. HotSlogs works with those numbers too, and I usally find it quite adequat; only healers and tanks shouldn’t be expected to do siegedmg.
And it was always said by the devs, that the PBMM would only be introduced to leagues up to Diamond, if I remember correctly.