While it is true that Brook is a bad rebounder, he isn't quite as one-dimensional as people seem to think. He's actually a pretty good defender (he blocks shots at an above average rate and commits fouls at a below average rate, key hallmarks of good defense; the past two seasons his defensive rating and defensive win shares have also been above average). Not an elite defender, but certainly a capable one.
He's not a terribly good passer, but he has great hands, and almost never turns the ball over. For a player that gets as many touches as he does (27% usage rate this past season), having a single digit turnover rate is a lot more valuable than you might think. He doesn't let himself get stripped, he protects the ball, and making safe passes.
Further, he is a monster on offense (and, in fact, for all his rebounding woes on defense, actually puts up above average offensive rebounding numbers). His TS% last season was a monster 62.9 despite not having a 3 point shot to bump his efficiency up. He scored a ridiculous 1.54 PPS and got to the line at a rate more than twice the league average, where he converted free throws at an 81.7% rate. Fun fact: Lebron's TS% was 64.9 and also made 1.54 PPS this past season.
That said, Brook is not a star. He is an above average starter that couples incredibly efficient offensive play with decent defensive play. Due to the fact that he isn't a good rebounder or passer, he needs the right system around him to work. I don't know if the Celtics are that system necessarily (and this is without even getting into injury related talk), but in general Brook is a good piece for a team that wants to contend.