If anyone is interested in how much rubber is left from the bottom of the tread, I just disected an Avon AV46 (rear) tire. It appears that in the center of the tire there is 2/32nds of rubber between the bottom of the tread to the first cord and an additional 2/32nds of rubber after the last cord although it is anyone's guess what the composition of that last layer of rubber really is.
It appears that the tire's ability to hold air is pretty good even after the wear bars and tread are gone but that is assuming that you do not run over anything sharp, not even a small rock with a pointed edge. A flat tire on a motorcycle is not that difficult to control -- until the tire's bead separates from the rim -- and then it is better to be lucky than good because it is nest to impossible to control a bike (at speed) that wants to swap ends.
I think you should change tires when the wear bars are apparent and I think in my state that the tire is technically illegal if 3 wear bars are showing on the same plane of the tire. One thing to keep in mind is that wear bar will interfere with water dispersion, which is why we have tread in the first place, so if you are going to be riding in the rain you probably should not ride past the wear bars.
