I used to have one of those BMW GS trail bikes equipped with ABS and I thought it was great. You could almost ignore the road surface when braking - the only indication that it was working was the strange twittering noise and a slight juddering through the bars. Of course this meant I just rode faster in bad conditions so any safety aspect was cancelled out by my own stupidity.
On the bad side fitting ABS to motorbikes is quite expensive, adds to the weight and the list of things to go wrong. ABS as an option adds roughly £800 to the price tag of an £8000 BMW which is a 10% increase. Adding the same £800 ABS to a £3000 scooter adds 25% to the price. You'd also need more electronics and a bigger battery. Having ABS on just the front wheel would be a cheaper solution as long as the rider is capable of dealing with a rear wheel skid. Certainly no-one is capable of dealing with a front wheel skid.
Oddly I haven't had so many issues with tyre grip as I used to with my big bikes. It's certainly not because I'm going any slower. It just feels that scooter tyres seem to have more grip that larger bikes but I have no proof behind this. Maybe it's because of less weight, less dive on the forks and the tyres run warmer.
Mike
PS: My BMW died a violent death when an uninsured driver shot through 2 lanes of fast flowing traffic and hit me head on. I didn't see him coming and I never got to even use my fancy brakes. C'est la vie. On the good side it got me into scootering as the BMW shop only had a scooter as a replacement bike when I recovered
PS2: MY BMW was bought in 1996 which makes it older than the touring BMW mentioned above but I never had the issues mentioned above - perhaps it was model specific. I would also argue against ABS being mandatory but simply cause I can't stand the idea of a nanny state. If you want to be totally safe then drive a hummer.