Under certain circumstances you want ABS off or disabled, as it dramatically increases stopping distances which can be very dangerous.
For example...having ABS enabled on an offroading truck is a bad idea, as you're dealing with hill climbs, loose gravel, and deep mud. It's very difficult to stop an ABS truck when descending a steep hill with loose gravel or deep mud.
Many winter accidents occur when people can't stop in time, and ABS increases stopping distances in deep snow. This would be another scenario where you'd want it disabled.
People who have been trained on non-ABS vehicles instinctively let off the pedal many times when ABS kicks in, and while incorrect, it can be a tough habit to break. Also, those who have driven an ABS car all their life and hop into a non-ABS can find themselves in trouble when they go to rely on ABS.
Remember, ABS has a net effect of 0 on safety.