Well... it might be a problem in my brain, or with the way I understand going aggro but... here's my problem with it.
You go aggro when you want to make someone do something and you've got some method of force or violence to threaten them with, right? And they can't or wouldn't immediately strike back.
So how's that work with one person against a group?
I'm imagining someone walking among the group, pointing a pistol at some random gang member, and saying, "Gimme yer gas or I'll perforate ya!" It seems like either someone else in the gang would probably take you out while you're focused on this one dude or... something. I certainly can't imagine, in this kind of scenario, allowing for going aggro against the group. Against that dude you've got the gun pointed at, sure. And maybe says, "The crazy bitch is gonna kill me! Just give 'er the gas, guys!" and maybe they like him and don't want to see him with a bullet in his belly, so they do it, or maybe they go, "Ehhh, fuck that guy, we don't care about him."
And... after typing all that out, I'm now thinking maybe it WOULD work just as well as a going aggro roll against the whole group. "Sucking it up" would just be them letting that guy get shot.
I guess the biggest problem in my mind is whether you'd be able to have enough over a larger group to be able to go aggro in the first place rather than it all just exploding and getting dog-piled. But that's something that would only be clear in the particular game.