I just looked at the quantitative evidence, and I am not sure it supports your claim.
Shooting:
-FG% for Powe is .519, Davis is .373, Scal is .413. So Scal is better than Davis there, but Powe crushes him. Now it is true that Scal has more range, but with Pierce, Allen, and Rondo on the floor with Scal, anytime he shoots outside it guarantees the C's will struggled to get rebounds. Adjusted FG%, which takes 3 pointers into account, is closer, but Powe is still on top of Scal, .519 to .511. Davis trails at .376
-Scals points-per-shot is 1.12, while Powe's is a whopping 1.60. Davis lags again at 1.05.
-If you take their averages and project over 48 minutes, Scal actually has the *worst* points statistics of anyone on the Celtics, at 11.7. Davis is next to last at 12.1, while Powe is at 20.7
So what is the shooting argument for Scal? Moving on to defense. . .
Defense:
-Powe gets 4.1 rebounds a game, 2.3 of them defensive, while Davis gets 3.3, 2.0 of them defensive. Scal gets 1.2 rebounds a game, 1.1 of them defensive. And remember Scal gets put in at PF or C, which means it significantly weakens the C's ability to rebound in a relative sense
-Davis averages .6 steals a game, Powe .32, and Scal .5
-Powe averages .55 blocks a game, Davis .21, and Scal .29
-Scal does have fewer turnovers, at .3, compared to 1.1 for Powe and .7 for Davis.
-In terms of 48 minute averages, only Ray Allen is a worse rebounder than Scal. That's right -- Rondo and Eddie House are both more efficient rebounders.
Therefore, what is the defense argument for Scal?
I'm not sure how to assess spacing. What metric works for that?
However, it seems like the *data* suggests a significant edge for "not Scal" in terms of offense and defense. You can argue the data does not "measure" Scal's real performance, but that's about as quantifiable as when we were told he had a "brain type" like Michael Jordan. . .
Anyways, so is there a way to build a case, by the numbers, for Scal?
By what metrics does Scal out-perform Powe or Davis?
shooting, defense, and floor spacing.