they do nothing about aggressive drivers, drunks or tail gaters.

No and they don't make sure our tyres are pumped up, or that we remembered our wedding anniversary and tied up our shoe laces. They have their limitations - they just spot people who are driving faster than the local speed limit.

True enough, drunks, aggressive drivers and tailgaters are menaces. But the danger from all those menaces is amplfied when the speed is higher, especially the last. My general policy if someone is driving too close behind me is to slow down, if possible. (I'd like to see an instrument which would vigorously alert drivers when they are driving too close to the car ahead for the speed at which they are going.)

As for people being distracted by looking around for speed cameras, or slowing down when they come up to them and speeding the rest of the time, those things happen because speed cameras are forced to advertise their presence. Drive within the limit and you never need to worry about speed cameras.