I sometimes wonder about the real-world usefulness of machine pistols. I mean, if you really have to spit out that much pistol lead at 1200 rounds/min., wouldn't you rather have the greater control and range of a submachine gun?
Also, there are several weapons out there that blur the line between machine pistol and submachine gun - the Uzi Mini and H&K MP7, to name two.
The catch about most full-auto pistols is, they often have a feature that disables full-auto unless a certain accessory (usually a buttstock) is attached. So, they're really not that concealable.
Granola said: Yeah, but isn't the Glock-18 specifically designed not to, hence its "military only" distribution?
It is, but it runs into the problem that such a small weapon sucks hard in the accuracy department when fired full-auto, thus pretty much necessitating the additional stock if you want to actually hit something.
Then there's the fact that, even with a 33-round magazine, the gun just burns through the ammo uncontrollably fast.
Basically, it's a weak weapon, good only for dick-sizing and not actually practical. Any field operative would much prefer other, slightly larger guns for better accuracy at medium range. The Uzi Mini is a good choice there, for being only a little bit larger than a normal 9mm pistol (not counting the magazine).