At the risk of inviting criticism here, I'll have to confess to the fact that I've had my L2500DT for over a year now, and I've never had it
out of 4WD.
I may be taking years off the life of my tires, but the fact is I've only had it on real pavement once, and at that time I didn't realize that 4WD was a bad thing on that surface. My property has only a little bit of gravel road, and I'm usually only on that part when I'm taking the tractor out or putting it away.
My normal runs, going almost anywhere on the property, take me over combinations of firm, loose and occasionally mushy soil. It's almost all on hills and uneven terrain, and it's not uncommon for me to spin a wheel or two. Also, when I'm using the box blade (a common occurrence), especially with the rippers down, the tractor tends to get a little light in front, and I'm convinced that in those cases 4WD improves steering.
I hate digging holes with my R1's, so it's comforting to have 4-wheel traction when pushing into a pile or doing various other FEL chores, too. In short, I just feel that it would drive me crazy to be shifting in and out of 4WD as often as I would have to if I were the truly conscientious type. I guess you can tell that I don't consider 4WD a frivolous marketing add-on.