Pierce will need to have his minutes parred down during the playoffs. That's pretty clear; he can't play for two or more OTs in a row like last year. Yes, if it goes 7, he can take some caffeine pills (plus cortisone shots) and do it all for a single game. But then, if that's prior to another conference series, then he'll need to play only 20 mins for that subsequent game one.
As his backup, TA can play at most, garbage minutes. He's a liability when an opposing team is making a run. Quis (or Sheldon) are bench guys, they can't play starter minutes in place of Pierce for a consistent duration of time. Scals in not a 3, during a serious contest.
Thus, Finley would make for a great addition, if only to add star power longevity to the 3 spot for the entire playoffs. What that does is create a two-headed small forward, with ample substitutes available to make everyone fresh.
At the same time, it keeps Scals away from the SF role and in his natural place to stretch the floor as a 4, when the situation calls for it.