I have a Sub-2000 and a Lone Wolf AR pistol lower with a 7.5" DI upper, both set up for .40 S&W. They share ammo and magazines with my pistol of choice.
The Sub-2000 was unbelievably cheap used, runs everything I throw at it, has okay sights and accuracy, actually rather stout recoil (for what it is), and folds to fit in a backpack. Dollar for dollar I think it was one of my best gun buys. It is light, small, and reliable, which is a difficult combination for most more powerful rifle calibers.
The Lone Wolf DI AR is super compact, has an EOTech on it, has all the nice ergonomic controls of an AR, and thanks to the DI mechanism, recoils like a dream. At least on paper, it has more energy than a 7.5" barrel shooting a 55 grain .223, it is much quieter, much easier to keep on target, etc. Due to it being a custom build, there are some toothing issues. Other than a M&P 15-22, there is not another platform I own that I can shoot as fast and accurately. Compared to similar-sized short barreled rifles, you much less noise and recoil impulse to deal with. Because the gas port is so big, I take down the BCG about every 100 rounds (at least now during break in) to get some jacket pieces out (still accurate out to 100 yards, the farthest I have tried), but since I know this, it is reliable too in that window. It is so controllable right now, that I am deciding if I even want to get a stamp and a stock for it.
Both of these offer features that rifle caliber platforms cannot match exactly, so I think there is plenty of reason to have them. It is icing on the cake that they share ammo and magazines with my pistols. If you want one, go out and get one. I have to say that the 16" barrel, non-folding versions would not make as much sense to me, but I am sure there is a place for them also...