Three reasons
1. I have 3 other ARs that I want to be able to mount the suppressor to.
2. A 10.2" barrel creates a hell of a blast inside the baffles of the can. Using a compensator saves the first blast baffle from getting as worn as it normally would without a comp.
3. Thread on cans come lose during rapid fire. Its a fact of life.
The only time I have ever seen a suppressor work loose during firing, were on those old mac's with the HUGE 7/8-9 threads.
But on an AR (or 1/2-28 or even 5/8-24 threading), I can honestly say I have never seen a thread on can work loose if it was put on snug to begine with. Even on my transferable full auto SMG. Matter of fact, when the can and barrel heat up, the threads will swell and be an even tighter joint. I can't count the times I've had to wait 15 or 20 minutes for them too cool off some so I could unscrew the can off of it.
Your #2 reason is a very valid point. But #1 and #3, not so much but as long as you're happy with it that way is all that matters.
Here is seven 36 round magazines totalling 252 rounds of full power 147 gr in under a minute and a half. As you can see, the silencer didn't unscrew and land on anyone's toes, nor any baffle strikes.