It's pretty straightforward, especially if you've played a Halo game before. People quit out if this magical number beside teammates names are not up to their standards without them actually seeing how they play. This was the worst in Halo 2.
That doesn't fix anything, people will still quit. TruSkill has been around forever and still matchmakes within even smaller ranges of that most of the time.
Honestly, from my experiences with close friends and randoms online, it makes a difference and people WILL do it, even if there's a penalty involved. People did it in Halo 2, they did it in Halo 3, and they'll do it again. I understand it can work as a positive reinforcer, but TBH the negatives far outweight the potential positives for the subset I mentioned.