ABS is generally fitted to high-end sports and touring machinery, especially BMWs. The people who ride these tend to be more mature, affluent and experienced - although the "weekend warrior" issue with high accident rates among middle-aged returnees has to be acknowledged. I think it's safe to say that you would expect the accident rate to be lower in this demographic.

There was also a link established in research when both seat belts and ABS first came out and in each case it was established that a significant proportion of early adopters were drivers who recognised the limitations of their abilities. I bet this is also true of ABS on bikes.

In normal road riding conditions, ABS can pull you up shorter and safer on a bike, although many riders take longer to stop because the juddering through the bars encourages you to back off the brakes. It prevents some of the recognised off-roading techniques and also prevents the classic "get-off" approach of standing on the back brake pedal and laying the machine down.

I cannot see any reason whatsoever why fitting ABS to the front brake alone on any bike would not be a universally-acknowledged safety measure. Halves the cost penalty, too.