I didn't read ever post word for word, so this may have been stated (I know it has been hinted at). As Clint Smith puts it "A handgun is for fun and rifle is for real". A handgun is better than nothing, but its best application in a fight is for fighting to get to a long gun! That being said, a MP5K (or other PDW) would be better than a handgun in a situation where you have no long gun nearby... but only slightly. Pistol caliber ammunition is still pistol caliber ammunition regardless of the length of the barrel it is fired through!
Like Iron and others have said, the HK94 (aka MP5) series of weapons has a well and hard earned reputation. They are reliable, durable, accurate and can be made to be more than silent! The series has had a long lived life as the prominent SMG for LE and military operations in many countries. But even with the various calibers and configurations offered by HK94 series, it is extremely limited by it only being a pistol caliber weapon. This limitation prevents it from being accurate at extended ranges and prevents it from even approaching the terminal ballistics of a good rifle caliber weapon. At 10 yards it doesn’t make much of a difference, but at 25, 50, 100, even 300 yards it does!
I’ll take a M4 configuration of an AR-15 over a pistol caliber carbine any day! Like others have said, the AR-15 series of weapons have a unearned bad reliability reputation. But, for the cost of a MP5 you can get an outstanding top of the line AR-15 and optics. If I am in a situation where it is all about CQB… like my home, then it is a 12g Shotgun all the way. Nothing says “QUIT BEING AN [censored]” quit like 8-9 32cal balls flying at your head at 2000fps!
The MP5 is a great weapon, but I don’t think I’d describe it as pretty… the Thompson maybe. Some 1911s absolutely! But, I’ll stick to my Sig P220 and super ugly M4gery and Remmy 870!