Winning consistently over time is the best indication of skill, maintaining high win rate at increasing tiers in a matchmaking system is the best indication of a progression in skill. Maintaining high win rate in competitive play is the best indication of real expertise.
Now other stats like kills and damage aren't very good data because they mean totally different things in different tiers and contexts. It is much easier to do high damage and kills for an ok player at lower tiers than it is for a good player in a high tier match. It's much easier to score good if your teammates are bad, and harder if your teammates are good, and so forth.
Competitive matches don't have higher damage/kill/whatever averages than then worst of the tier 5 scrubfest matches because the damage it takes to kill the enemy remains about the same, so looking at those two matches there is no way of telling that one is full of really good players and the other is full of really bad players. Therefore damage and kills and any other in game performance stat can't be used to measure skill.
Now it's true that group queue win percentage can be "inflated" by consistently running with a team of players that are a lot better than yourself, I'm not convinced this is a big factor though. Planning for a match by coordination of mechs and such things is a part of your skillset so there is nothing really wrong with having the ranking system reward you for that.
The problem with MWO's current ranking system isn't that it measures the wrong things, but rather that there is no zero sum qualifier regulating how many players can be in each tier. This means everyone can be tier 1 in theory, the "XP bar" problem. It would be much better if tier 1 was capped at for example the best performing 10% of the players, tier 2 the next 15% and so on, if people improve to surpass you it should be possible that you are pushed down in tiers. That is a huge problem, but measuring skill mainly by winning is the correct way and not something that should change.
Edited by Sjorpha, 14 June 2017 - 11:21 AM.