Quoted:
I believe the idea is to bring a 10.5 inch to the minimum 16 inches. I suspect the primary reason is asthetics, with no intent on ever shortening the barrel legally. The other reason I can think of, is to be able to go ahead and build a rifle, while waiting on ATF paperwork for an SBR. I think there is a thread here somewhere on how to remove a permanently installed 5.5 inch flash hider when the paperwork comes.
I have been considering this option for a 9mm build, with the understanding it will be a long time until I am able to afford getting SBR paperwork approved.
As nicholsmf mnetioned, it's a 11.5" barrel once you include the 1" or so of threaded section from the 5.5 FH.
My only positive thing would be that it could be used to replicate a Resto build. I forgot the name of the Mill rifle, but there is one that had the longer hider on it.
Beyond that, I personally don't like them. You'd be better served with a pinned 14.5" barrel and Smith or Phantom on there.