Yeah, there are long discussions about this subject!!! Heheheheh. Everyone has their own ideas. Here's mine.
Use 4wd when needed. Disengage when not needed, regardless of type of terrain.
MFWD designed machines bind up when engaged either when running straight or when turning. This binding will cause wear. Examples of MFWD are our tractors with different sized tires front/rear and the mechanical ability to engage and disengage.
4WD designed machines do not bind. Examples of 4WD equipment are center pivot large AG tractors or loaders. 4WD equipment are generally engaged at all times with no ability to disengage.
I do a lot of work with my Kubota in 2WD. I notice a definite increase in drivetrain noise and a loss of turning radius when 4WD is engaged.
My final point. Do you engage 4WD in your pickup every time you leave pavement?? Treat your tractor similarly.
Now let the arguments begin!!!