I don't find ATVs to be an "unfair advantage".
I do find them incredibly annoying and destructive, though.
In the last 15 years, I have known or encountered a total of three people that kept their ATVs on designated trails, and didn't have 350 decibel resonators on their exhaust. The rest were:
A) Ignorant <expletives> that didn't bother to read the "road closed" signs, and still didn't care when they were informed that the closest legal road is 4 miles away.
B) Just plain <expletives> that KNOW they're breaking the law and irritating everyone on the mountain, and actually go out of their way to make things worse - all while showing how 'super bad ass' they are, by revving their ATV into oblivion, and tearing up the mountain.
Where I hunt Elk, there is a sand stone precipice at the peak of a mountain, that overlooks a huge valley. The closest ATV trail is 2 miles away, and most of the "hiking trails" are actually closed due to erosion issues (ATV-induced, naturally). Last year, an ATV rolled over another hunter before going off the top of that precipice, falling 600 feet, and tumbling down a 70 degree slope - because people are too damn ignorant and arrogant to obey Forest Service regulations.
If you're a responsible ATV rider, that doesn't have a 'jetted, piped, and stroked' bike screaming around the mountains, I don't have a problem. But give me excess noise, or one trip down an illegal (or non-existent) trail, and I have zero respect.