And spend most of that ammo up in the air vs. on target.

Look at the case of criminal negligence where a young boy was handed a Micro-UZI to shoot and the path it traversed ended up shooting him in the head.... Or the various combat veterans in this discussion attesting to the limited utility of full auto in an AR-15 pattern rifle. Or the Mystery Science Theater 3000 episode where one of the kibitzers was shouting "Short, controlled bursts!" ^_^.

In a firefight between two militaries, it is not more dangerous. The danger is that the regular police that only carry sidearms would be seriously outgunned.

Not really. The point is that in fully automatic mode, you can blow through a backpack full of ammunition in about a minute, probably not hitting anything because the continual fire and your efforts to control the gun are throwing the barrel all over the place. Now the cops are standing there, they're pissed, and you're out of ammunition. If you were a typical "crazed killer", this is the point where you'd shoot yourself, but you just sprayed all your bullets down the street.

Bump firing the rifle would be inaccurate if the shooter went through the whole clip with one or two pulls. This stock attachment allows for reasonably accurate burst firing.

A gunman with an automatic weapon will not necessarily waste all of his ammunition with continuous, inaccurate fire. With very little training, he can learn to burst fire. This capability blurs the line between a military and civilian rifle.

There is a video of the stock in action here [1]. It seems to me that someone with limited training would still completely outgun a regular police officer armed only with a pistol.

[1] http://www.youtube.com/watch?v=dvLt8-Wf7r0

... so? If someone is well enough trained to fire controlled bursts, they're probably well enough trained to be just as effective double tapping in semi-auto.