Red you seem to never fail to offer quality, well thought out and original OP's. I have never personally been a fan of bot replacement of players in online match making. The reason is that a bot would most likely play to a statistical standard of the player he is replacing, assumably a perfect representation. Humans are, well humans. We get distracted by something in our gaming location. We get frustrated and have bad streaks, forget to replace low batteries, girl friend/boy friend walks in the room and tickles you, etc, etc. One current example I can use is Full House Poker were I have invested a very large sum of my winnings into a game, player lags out or quits, now I am playing a bot. Personally I play MM for the tactile experience of matching up against other players from around the world and seeing what comes out in the wash. I may get out BR'd in a slayer game, but know that forcing another player at a critical time of the match, weakening his shields or forcing him out into a more vulnerable position for team mates attack which may give us a tactical advantage in that part of the map or acquire the desired power weapon from their grasp. I don't see a bot being able to have that "intuition". Players coordinate attacks using a wide variety of tactics and having a bot doing it's own thing may be worse than being a player down, which is not always a disadvantage.
I am fine with the "Submit Player Review" feature to help ensure limited chances of being paired with said player with the possible addition of allowing a player to have more than one bad "Player Review" be submitted on them by a player if repeatedly matched with them. This would, after say 3 bad reviews, guarantee never being teamed with them again. Eventually the quitters would end up being paired with each other depending on the overall number of quitting reviews.
On a side note, bots in custom games would be a great way to train, especially if somewhat programmable. Setting them up for slayer or skirmish and testing your personal skill or teams tactics would be a great advancement in gaming. Using bots to test forged maps for and weaknesses, playability in general or for determining shooting lines would be a great resource to a forger. Imagine testing a map segment for feasibility before committing hours of forge time.