My guess he bought it because it was used on Stargate and various other SciFi movies and it is a cool looking gun, without that dumb 16" barrel, and the salesmen sold him on 5.7's reputed AP capabilities, failing to mention that most 5.7 sold to civilians is both expensive and anaemic compared to NATO AP loads.
Sure if you reload (and if you shoot enough I'd recommend anyone to get themselves a bench, the initial cost is more than repaid by savings if you're out every weekend or even a couple times a month) you can get hardened rounds and tailor your loads but Pats a moron, he'd likely blow his face off trying to hotload, so what he's left with is an SMG thats single fire, has a stupid overlong barrel negating the whole point of a compact, and that takes 50 round magazines of overpriced, anaemic ammo, that is still never the less likely to over penetrate drywall and endanger his neighbors.
If he wanted something cool looking he'd be better with an X95, I've got one and for a Bullpup its fucking great, I still prefer my DDM4v7 but once you put a trigger pack in it doesn't even have the weird creep you get with bullpups, it's easy to switch out barrels to run either 5.56 or .300BLK which with a suppressor and subsonics will save your hearing.
The PS90 though is just a weird choice and shows Pat doesn't really know his stuff, as if all the weird Chinese shit on that shitty Glock didn't already say that loud and clear.