I have heard this before, but I cannot understand why you think this is the case. A mountain bike weighs about 30 lbs, a motorbike weighs about 250 lbs. Now compare the power of mountain bike vs dirt bike - going up a crazy steep slope (lots of erosion) is the turn of a wrist for the dirt bike, but completely impossible on a mountain bike. Nevermind that you can ride a dirt bike for as long as you feel like it, but on a mountain bike you are probably not doing more than 5-10 miles in a day.
Just anecdotally, I have hiked on a lot of mountain bike trails that see hundreds of riders every day and I have hiked on motor bike trails that see maybe a handful of riders on a busy weekend. The motor bike trails are always more rutted-out. Have you experienced something different?