Is there an advantage to running -4? I strongly suspect that the answer is yes, at least on some tracks (and Jim seems to agree). Is there an advantage to -3.5 instead of -3? Absolutely. So we have two problems:
1. If my subframe won't let me get to -3.5, I need to start swapping parts, or I am at a disadvantage to legal cars.
2. If my car can get to -3.5, but the track requires -4, I am at a disadvantage to illegal cars.
Problem 1 can be fixed with a stack of parts and a ton of time: not a great solution for a weekend warrior. Problem 2 can be fixed if tech has a complete set of suspension jigs with published dimensions and tolerances for each part and pickup point: not gonna happen.
Teching for slotted control arms is easy. Teching for intentionally bent uprights is practically impossible. If -4 is faster, people will find a way to get there. We can eliminate the problem by simply allowing slotted arms. It costs nothing and only takes a few minutes. People aren't going to suddenly start running -6, or even -4.5, because it won't be faster. But people with tweaked subframes (or tubs!) will be able to get the numbers they want. And none of us will have decide if we're willing to bolt on illegal (but non-tech) parts.
I'm reasonably certain that some very fast cars are already running intentionally tweaked uprights. I REALLY don't want to do the same and I REALLY don't want to let those cars have an unfair advantage. This is exactly the same problem that led us to allowing offset bushings in the front. I understand the worries about rules creep, but this is honestly a no-brainer for me.