I agree, but I think you have to realize that Ainge made this move primarily to help with certain matchups in the playoffs.
Remember Gerald Green last year and his great impact in that Bulls series?
Monroe could certainly help in spurts dealing with the likes of Drummond, Whiteside, Embiid, etc. in the playoffs especially in the early rounds.
Although he still looked a bit shaky, I thought he did well against Drummond, a guy who usually kills us with 25/25 games, but in the last game he only had 15/15 against us (which is great but not as exceptional, and we can take that).