The original PDW 5.7X28mm concept , which dates back 10 the late 1980s, was indeed a highly specialized beast. The FN P90, and later the FiveseveN, was intended as a personal defense weapon for REMFs in a theater of operations more like "conventional land warfare" than low-intensity conflicts against non-uniformed insurgents like we seem to be fighting these days. In such a scenario (I.E. ground combat against the Warsaw Pact in Western Europe), enemy soldiers would have been soldiers from standing national forces, which would likely be wearing some sort of body armor. Rear echelon types were currently being armed in the main with pistols or pistol-caliber submachine guns, which, with very few exceptions, are incapable of penetrating body armor at all. This left the support troops practically unarmed against combat-arms units. The PDW concept was born of a need to find a weapon system that would penetrate the armor, but still be very small and lightweight as to not overly interfere with the primary mission of the support troops. Basically, any terminal ballistics against armored soldiers was better than nothing, which is what the 9mm cartridge provided.
Now, we gun people being the way we are, we quickly want the same sort of toys the soldiers get to play with, especially when they are funky and sexy looking like the P90. The feds certainly didn't want the armor-piercing rounds in the hands of us mere peasants, so FN developed loads that would not pentrate body armor. What resulted, IMHO, was neither fish nor fowl, too slow to damage with rifle-like cavitation and fragmentation, and without sufficient mass or diameter to create much of a permanent wound cavity. With the original armor-defeating round, its a great concept which has found a definite niche in law enforcement and various armed forces. (Although the HK MP7 and its 4.6X33mm round seem to be doing better in that regard worldwide). With the "safer" ammo, it's pretty much just a novelty, albeit a really cool one (we love shooting our demo PS90 when we can afford it).
DanO