Dumb idea if you ask me. I want complete control over when I want lockup or not (sometimes a controlled lockup is the best and fastest way to get through a trail section), and modern disc brakes offer really good control and modulation. To me it's a solution to a non-existing problem, and in fact will take away a level of control from an experienced rider.
the speeds involve don't generally warranty it. Might be useful on ice, snow or wet slippy stuff though! I guess the likely additional cost would rule it out for most.
Cars/mcs fitted with ABS also have brake bias and traction control. ABS on its own is pretty crude and is more likely on a mtb to upset the balance of the bike under braking than help it to stop quicker.