Why ABS could be bad for you when off-road

ABS was designed to keep wheels from locking up. ABS was designed to make driving safer (in most circumstances).

Locked up wheels can no longer steer the vehicle. For example, you would not be able to steer around an obstacle with locked wheels - with wheels locked you would slide straight into it. So, even though you are able to turn your steering wheel - the vehicle can not follow this command when the weels are not rotating. ABS keeps wheels rotating.

Again, ABS makes driving safer.

Not so on a dirt road!

Sand and gravel under the tires act like tiny ball bearings when you step on the brakes. As a result there will be very little friction/traction between tires and ground.

Therefore all four wheels want to lock up immediately.

ABS would keep them from doing so.

So you keep rolling and rolling, and rolling, and rolling. You might be rolling for too long and your stopping distance is going to be that of a container ship. Too long to be safe. You might even drive off a cliff.

Here is what you get with ABS off: When you hit the brake good and hard the wheels will lock up - the tires will immediately begin to dig in pushing against sand and gravel - this builds little berms in front of each tire, which very effectively slows down the vehicle. As you see, on dirt locking wheels can be good for you! Picture a cartoon horse stemming all 4 into the dirt after a fast run.

In fact, stopping without ABS on sand or gravel can be so brutal that you better have your seatbelt on - otherwise you will end up with your face in the steering wheel. Been there.

To deal with the negative effects of ABS on dirt, some manufacturers had installed a kill switch for ABS in their full time four wheel drive vehicles (for example early 90's Mercedes-Benz G-Class) but recent changes in the law prevents them from providing this option any longer. Seasoned four wheelers have either installed their own kill switch or simply pull the ABS fuse before hitting long stretches of dirt road.

Vehicles with part time 4WD don't really have a problem with ABS - ABS does not work in combination with part time 4WD anyway.
Also, on some vehicles you have an option of using full time 4WD/AWD or part time 4WD. Its up to you to chose the right setting.

More examples

By the way:

Aside from ensuring that you can steer your vehicle even under heavy braking, ABS will keep your vehicle from fishtailing.