You can get more suspension travel without being concerned with CV's and binding. You also have fewer moving parts to break. For the most part you are moving too quickly for 4wd to make a difference but there are exceptions such as water crossings, deep silt and sand where they do get stuck.

In short course racing (CORR) the PRO4 (4WD Open Full Size) kick the crap out of the Pro2 trucks with itentical engine and chassis in head to head competition because of the advantages of what is esentially an all wheel drive system.

There are some very competitive class 7 (minitruck) desert vehicles that are 4wd that use less suspension travel but have the 4wd to avoid stuck situations. It's kind of like having a second kidney. You don't really need it but if you do it's nice knowing you have it.

Then of course there is the Herbst Trophy Truck that is 4wd.