The problem is the 4.5 db drop in level when you engage the quad buss pan and then if you are using it the way you are suggesting and movement of the pan know changes their overall balance.
Let say I have a mix going and I'm liking the balance but I want to vox, guitars, and key to interact a bit more. So I engage the quad buss pan to send to the back buss on those channels. Now my level of the vocals, guitar, and keys in the front buss have changed. Not to get them back to balance I need to make sure I push up each fader 4.5. So I do this. Now I don't like the vocals being in the rear buss which is getting compressed, but I like the guitars and keys there. No I disengage the quad buss pan on the vocals and now they are 4.5 db to hot. So not have to pull them down and rebalance everything. I can't see that workflow working well.
If I did the mod, I would engage the quad buss pan and my mix would stay the same. Now there is an exact copy going to back buss which I compress and bring up on the desk in two channels. Anything that I'm not feeling sounds good in the back bus, just get removed by disengaging the quad pan. Easy.
Let talk and example with drums using the way i want to modify it. You get your levels happen, but you want to parallel compress the kit. Just engage the quad comp on the drums you want to parallel process and there is your compress buss to bring back and blend with the kit. Super simple, but also super effective.
if the group pans followed the main, then this wouldn't be an issue and we could just use busses 1-3 to accomplish, but they don't.
I am hoping you are following me on why this would be beneficial.