Armchair experts missing the point as usual.... No one is claiming that you should run 4wd all the time on every surface. That is obviously bad practice.
But as to the argument of "it will damage your tractor" or "you're wearing out the 4wd components"
No.
They are designed to withstand being ran in 4wd for the life of the tractor because there are people who run them in 4wd all the time. Saving wear on the front axle is hardly relevant if the rest of the tractor is ready for the scrapyard before you start having trouble there.
And for the average person who owns a compact tractor for hobby use and puts 50-200 hours a year on, it is a complete non issue.
The mfwd system is switchable because it tears up tires on hard surfaces and tears up the ground on loose surfaces, not because it is "bad for the tractor" to use it.