I'm of the opinion that 4wd should only be used when needed, or when traction is light enough not to stress things. Running it all the time is just more wear and risk of breakage on tires/front drive components/lawn/etc. Heavy loader loads in my opinion are not good reasons to use 4wd unless you need it for traction in slippery surfaces (not because your rear wheels are light), you need more ballast in that case. The rear axles on every tractor are built much beefier, and are much simpler than the front, no need to stress them just running around. My tractor has no power steering and it only takes one turn while mowing to realize I've left it in 4WD after coming through a wet spot, the binding in the drive line is significant on dry grass, and this is on a 1400lb tractor with no loader on. Imagine the stress when you get your power steering working against your drive line in a large tractor with the loader on.
Obviously if you're running with turfs on wet grass on hills or through loose dirt or mud, the risk is much lower and leaving in 4WD is beneficial so you don't spin the tires, but anytime it's dry, it should be 2wd. Not just pavement.