Unless penetrating a Level III Kevlar vest WITH Trauma Plate through and through is useless, plus you can carry it in your jacket or coat...
Secret Service Likes It. Wonder what they know? 
The SS have needs that most of us don't, besides they often have ALOT of fire power near by. its not uncommon for them to have a 7.62 mini gun in a SUV/ van close by.
I mean, they need something that can be hidden in a jacket, can stop a armored target, has a high rate of fire, holds lots of ammo, and won't go thru a bunch of walls and kill some one.
I really can't think of another group that has those needs.