Have not had to use it in rain before, but I do use 4WD whenever I'm pulling the camper in someone's yard (we do a fair amount of of yard camping at friends & family homes), or if I hit a long dirt road.
While not always necessary, I like to make sure everything works. In fact I usually test out my 4WD at least every couple months.
I've known a couple guys that hadn't used their 4WD in years, and when they really needed it, they find their actuator had seized up so they couldn't get the front axle to engage.
Just don't drive down dry tar.