As just a normal dude who currently no intentions of being combative with my firearms, I've never really seen the need for an sbr.
Even with this legal loophole, it just doesn't do anything for me. The best arguement that's applicable to me is the "truck" gun arguement. But even there my 30-30 has worked fine so far...
Some people want them, some people don't. One reason would be to have a shorter gun. Here's an example:
Both guns are chambered for 300 AAC Blackout. Both guns have a QD mount to accept the suppressor. The top gun has a 16" barrel. The bottom gun has a 8.2" barrel (SBR tax stamped). Even with the suppressor (tax stamped) attached, it's shorter than the 16" barreled rifle. With the suppressor removed and having a short barrel, it is easy to pull that rifle out and drop a pig or coyote on the ranch.
In some states like Indiana, they can't hunt with "high power" rifles. They can use a rifle chambered with pistol ammo and they can use a pistol chambered with rifle ammo (any pistol). So for them, using the Sig SB15 brace on an AR pistol is the only option if they want to use a caliber like a 6.8 to deer hunt.