ABS on the more modern bikes is fine in gravel, at least for a road bike. I have never come near having the ABS kick in on gravel roads, as you can brake pretty hard if you try, and the traction control for the rear wheel is hard to get kicking in (I know the ST1300 does not have traction control). I tend to try and not go near that limit in gravel and the wet through old fashioned restraint, but you can go a lot faster than you think. Combined braking also works fine, at least the Trophy system does, and I think even better than rear brake only.
On a dedicated dirt bike than having the ability to disable ABS and traction control can be a plus, but only for a rider that can use those extra limits to control the bike balancing the front and rear by controlled power. For most punters ABS is a far better bet in all conditions.