Well, I have all three, but I need to build the HB Industries K version for my Scorpion when I get home. I have the Sig MPX-K, CZ Scorpion, B&T GHM9, CMMG Banshee, RONI STAB for my G17, a few AR pistols (couple home built, DDM4V7 pistol, a 300 BO AR pistol), and an AK47 PAP pistol with arm brace.
They are all fun, but "practical" or "suitable" really depends on the environment and purpose. I will state up front, none of these PDW type "pistols" are a replacement for a concealable pistol; nor are they are replacement for a carbine/rifle. They are very much niche firearms.
As to their application? They are less concealable than a conventional CCW, but quite often more concealable than a rifle. Pistol caliber versions are simply large handguns, but they do offer vastly better accuracy, a much more stable platform for fast follow up shots and multiple engagements, a better platform for lights and sights, and stretch realistic engagement ranges out to 100-150 meters with much better accuracy than a conventional handgun.
The rifle-caliber pistols do offer better ballistics, but give up a lot of range and penetration potential with the shorter barrels. Noise and blast are pretty severe in confined spaces. I'm still quite impressed with accuracy even if the terminal performance is degraded. I'm easily engaging silhouettes out to 200 meters consistently.
Both are good choices for suppressor hosts. I wouldn't use a rifle-caliber pistol for HD (except maybe the 300BO when suppressed), but I would quickly grab my MPX-K over a handgun or even the shotgun. For patrols around the house and barn, I really like the compactness, lighter weight, maneuverability, accuracy, and performance for these PDW-like "pistols".
Other than lack of concealability (they're not a realistic CCW), I see both pistol and rifle-caliber "pistols" with arm braces as a very viable choice from about 25meters out to 150 meters when it comes to the "why" between a conventional handgun or rifle. They're not a necessity by any means and I wouldn't choose one over a handgun for CCW, and I wouldn't choose one over a conventional rifle for serious social engagements or combat; however they do fit their niche extremely well and are a lot of fun to boot.
ROCK6