So if am matched with an extremely underperforming teammate, am supposed to just believe that it he just had a bad day ?
Good question. Depends whether besides your own BPR/Skill rank whatever it shows the range the system is using when searching for more players. If it's a loose range (say, you're the equivalent of a 25 and the game is searching +/- 15 ranks) it should be fairly easy to tease out who are the weaker links just from the game's stats. With joining in session active in some playlists, though, you may have no idea except going off of what the range was in the pregame lobby, which may have changed in the 4 or 5 minutes since starting the game. And nothing is going to prevent scrub friends from joining.
Of course, with Waypoint, it's likely that finding out the player's normally "invisible" rank will just be an annoying chore rather than having it up front, which doesn't really deter boosting
as much as it initially sounds. I'm sure there was or were kids that wanted to buy an account that had a 5+ k/d or went onyx in the last Arena season, whether or not they knew what that entailed (going into MM Grifball 2 gamertagged players and 2 guests to a team) or that their Onyx might not carry over.
So it still begs the question: why not just open up the skill rank to different play styles and the new medal/point system so you have more getting to the middle quicker, versus making it only slightly less of a TrueSkill black box than Reach, and removing Arena?