Everyone's got an opinion on this but here's mine:
I was hoping they'd wait until after free agency, when I figured one team that had struck out on big-name free agents would call about Kemba. Maybe the team was impatient, or maybe they put feelers around the league and didn't have faith that there'd be any demand.
I have no problem with where the Celtics are at PG now. It will be Smart, Brown, Tatum, another wing/swing if not Fournier, and a big. It will be a really competitive team defensively that can switch everything and body up physically. I also think Pritchard can give you 20-25 good minutes in his 2nd year. But anyway, this team will play bigger and more physical next year.
I like adding Horford to the locker room, even though I'm not sure he's good enough at this age to be a starting center on a top-tier team. He may backup a healthy Timelord and play 20 minutes a night only.
Does this help the luxury tax conundrum? If I'm not mistaken, the Celtics still can't re-sign Fournier now that they've swapped Kemba for Horford, and still be under the tax threshold. Perhaps picking up Brown instead of the #16 pick helps on the edges. To get under the tax, perhaps the next move is to find a home for Tristan Thompson on a team with cap space, since you've now got Timelord, Brown, and Horford at center (plus Grant Williams, plus perhaps Kornet on a vet minimum). Adding Brown is nice also in case the team's future big-package move to pick up a star includes Timelord.