Necessary is subjective, but adding protection against thrown up gravel is a wise move if you will travel extensively on Alaska roads. Especially if you tend to drive aggressively or at higher speeds. A lot of risk can be avoided simply by slowing down, slower than you would drive on roads in the lower 48. If you are cruising along a nicely paved two-lane road at 55 and hit a patch of gravel where a washout was repaired with temporary fill, gravel will fly everywhere. Including towards any oncoming traffic.
We found our towed car to be more at risk than the motorhome itself. Our motorhome threw up gravel from the back wheels even at lower speeds.