In reference to shooting from the choppers, I think a southern california police outfit (Maybe Riverside Sheriff I think?) trained their pilots to shoot MP5 submachine guns from the aircraft after having a pilot shot while trying to follow a suspect. Saw a video of it, looked pretty nifty... and while I doubt they've shot anyone since, I think it's better to be prepared than left hanging when you need the capability.
I've shot a rifle from a helicopter as a passenger, and in some situations, I think it could be very effective... and as that big tank chase in California in the late 90s shows, police are usually the only immediately available asset when something goes crazy... so those SWAT units better be prepared.
Mike