I think you hit the nail on the head. I personally think, and I could very well be wrong, that the popularity of the sig brace is in the fact that it basically came as a work around to the law against pistols with buttstocks, because it *could* be used as a brace attached to the wrist, with the majority of those who purchased it intending to use it as a makeshift buttstock. The gunstore I worked at last summer got one of the ar-15 pistols with a brace on it in, and literally everyone who picked it up put it to their shoulder, and most remarked on its legality. It's the same with the popularity of the sliderfire/bumpski stocks as an alternative to full auto. It's not 100% the same thing, but it is a substitution.
I personally believe that sbr's, machine pistols, and the like should be legal; but as the current law sits, I totally understand the ATF's position, and actually feel they are being generous. They're giving gun owners enough rope to hang themselves with, it's up to the purchasers of the brace to hang themselves. I'm going to just go ahead and say that all of the uproar is about people being upset that they cannot legally circumvent the law now, which sucks, but I wasn't surprised it went this route. In fact, the only thing that does surprise me about this entire thing is that it was ever permitted in the first place and allowed to run for as long as it did. Where the slidefire/bumpski stocks were an attempt to circumvent the laws against FA, they might have produced similar rates of fire, but not for any sustainable period and without anywhere near the accuracy, since the weight of the gun is no longer firmly in the shoulder weld and recoil is less controlled because of it, the brace, on the other hand, was a more effective replication of what a buttstock provides for a pistol in the control of recoil forces with 3-4 points of contact and being able to effectively fire the weapon while holding it closer to the center of gravity and reduce arm fatigue, plus the benefits to sighting that comes from all of the above. I spelled it out. Again, I don't agree with it, but I totally see where they are coming from, and it currently IS the law. What we need to do, and I know this is likely not going to happen in the near future, is get the laws on SBRs/SBSs reversed.