The LE version is the only version that NFA rules would apply to, and probably the most useful. I have seen other versions of a handgun/carbine adapter but this one looks promising because its universal.
Other than the LE version, and even then its still a pistol, I don't see much need for one in my arsenal although it would be cool in certain applicaions.
I'm thinking a FN in 5.7 would be cool, with a silencer and red dot, for sage rats.