I have not had actual experience of this situation (gravel, etc) but I've heard of others who have and according to them, this is one of those kind of situations where ABS really shines. The difference between ABS releasing the brake and the rider doing it is the ABS does it very quickly, when necessary, and then applies the brake again whereas the rider would be inclined to release either too long or all together.
As for ABS in a cage, from personal experience I prefer it in slippery road conditions, especially snow and ice.