Perkins playing defense against the other teams backup center would be a waste. You want your best players to play the other team best players. He'd also be a terrible fit in an offensively challenged bench and paired with guys like Powe and Tony Allen. And the bench is also in need of what Scal brings. Besides that, Garnett defending centers is a terrible idea on the long run. But, most importantly, Scal should not even play in a 8/9 men rotation.
But then again, you have to factor on how Doc uses his rotations and on how other teams rotate their players. We often find ourselves having to use Baby and Powe to guard the opposing starter because the way Doc rotates players is in blocks of playing time. So Perk might find himself starting to play against the backup, but Doc can keep him in the game for when the starter comes back.
The whole "best players on the floor" theory doesn't really fly with me. It ignores other factors. Various teams have used defensive specialists in the starting unit, for example, even when better players are on the roster.
The Garnett thing is quite valid, but I'm thinking more of strategies that win games and bring out best outcomes in individual games. But surely, you don't want Garnett banging against centers often. But then again, the way Doc rotates players, Perk should be there when Garnett comes back on the floor for a good period of time. Also, for those that want Posey as our PF to spread the floor, last year he was often playing in the 4th instead of Perk, so Garnett had his great share of defending centers. It's not really unusual. But a reason of solely wanting to protect Garnett is quite valid.
And again, just because I like him with our starters, it doesn't mean I like him as a rotation player. You might like what he brings to the bench, I really don't care much for it. Circumstances change, and as the circumstances change my opinions of how a player should be used changes, so I can't agree with limiting ourselves to the idea of him not playing in a 8/9 men rotation. Circumstances change, needed roles change, usage of players change. The scenarios are simply different and my opinions on the player changes regarding the different scenarios.