Hybrid Pistol/Rifle-Type Weapons

Anyone familiar with sub-machine guns knows, of course, that vertical foregrips have been around for a long time, the Thompson sub-machine gun being a well known and early example. Another, more recent example, is the H&K MP-5K.

(Source: http://www.af.mil/news/airman/0106/00_PDFs/50-51_Air_Base_Defense.pdf)
Although the illustration above shows the MP-5K with a folding stock, the typical model had no stock, but was aimed and held by using the two grips. It's an interesting idea--a pistol type weapon that can be steadied with a foregrip. It is my understanding that U.S. law, unfortunately, prohibits attaching a foregrip to a pistol, without going through the messy and expensive process of obtaining tax stamps from the ATF. So, it was much to my surprise that I came across descriptions of not one, but two such weapons today.

The first is a hybrid pistol/rifle weapon based on the AR platform manufactured by Franklin Armory called the XO-26b.

It has a short-barrel, but retains the buffer tube of the standard AR system. As I understand the article, because it lacks a stock, it isn't subject to the restrictions on short-barreled rifles; but because it is designed to shoot two-handed, and long enough to not be readily concealable, it is not a pistol either. According to the web-site, Franklin Armory has a letter from the ATF approving the configuration.

The second are photos of Kel-Tech's PLR16, a "pistol" shooting 5.56 NATO, set up with a front rail system and Magpul AFG (angled forward grip). The blog's author links (in the comments section) to a letter from the ATF approving the mounting of an AFG on an AR-style pistol.

The question is whether such weapons fulfill any useful role. Never having handled or shot one, this is pure speculation on my part, but I think they would. Here is my hypothesis: More powerful than a handgun, but lacking the stock and longer barrel of a rifle, I can see a hybrid weapon being useful for self-defense inside the home (especially trying to squeeze down a standard interior hall or through doors), or even at short ranges outside. They seem like they would retain much of the handiness of a handgun (there is a reason why hostage-rescue teams, such as Delta-Force, rely on handguns over longer weapons inside buildings), but potentially offer more stability than a handgun. Hopefully, I will someday get the opportunity to test my hypothesis.