At the time, it was clearly the right move. Kemba was coming off of an All-NBA season and Terry had clearly worn out his welcome. Of course, with Kyrie gone, it could have been just fine, but changes needed to be made to that team and trading out the disgruntled Terry for Kemba was a no-brainer. It also solidified to us and the rest of the NBA that Boston doesn't suck and that players still do want to come here.
In hindsight, I would say I would rather have Terry at half the price of Kemba. I don't think Terry is better and he doesn't draw the attention of defenses like Kemba does, but they are close enough that it would make sense to have the substantially cheaper player. I loved Terry when he was here, but I will say Kemba's personality is million times more uplifting to the rest of his teammates so I don't want to undersell that part either.
Funny enough, Philly would have been much better off signing Terry instead of Al. Who would have thought 'overpaid' Terry would be the guy everybody else wished they had signed?