I know moyorcyles have been and are very popular for off road use. I've watched the Baja 1000 and know their capabilities. I don't own one, never have. That said, I'll spew a little of my own concerns about them.
Around here everyone complains how the 4 wheelers tear up and damage the enviornment. 2 wheelers do the same and maybe even worse. The Forest Service allows 2 wheelers (motorcycle and bicycles) on the hiking trails around here. I hike a lot during the summer to keep in shape. I've seen the damage both do to the trails. Lots of the motorcycles are not street legal and some don't have mufflers enough to make them less noisy...they are loud. Just the opposite with Mt bikes. I've almost got run over by them because you can't hear them. They come down hill really fast and are on you before you know they are there. thr trails really get chewed up from their knobby tires.
IMHO if 4 wheelers don't belong off road, neither does the 2 wheelers. I know this will really tick off some of you, but this is how I feel. I have my body armor on!
CC, I understand your concern and in a lot of ways, agree with you. That said, I think that, like a lot of things, the bikes aren't the problem, it's the owners/riders.
While doing some research, I came across a motorcycle forum where some of the members expressed some of the same concerns as you and actually want to get their bikes as quiet as possible and have a real distaste for the riders who go tear up the trails and give them all a bad name.
As far as the issue of quiet bikes sneaking up on you, it's up to the rider to make sure that they don't make themselves a hazard to hikers.
But, there are the same kinds of issues with inconsiderate or downright irresponsible hunters. I've talked to several people around here who would like to go hunting, but refuse to go around here because of bad experiences with other hunters, either tearing the place up, leaving huge messes behind and/or threatening the safety of everybody around them.
Personally, I'm only considering the bike because I don't know if it's my age, the weight I'm carrying, the altitude or a combination of the three, but hauling a pack out in the mountains here is kicking my butt. As a result, I can't get very far in and when I get back, I hurt for weeks. I have never been in so much pain as I was a few nights this past month, there were a couple of nights that I couldn't get to sleep without some Advil because I hurt so bad.
As I mentioned, I would love to get a horse(s) (or mules), but I don't have the time and money to properly care for a horse, and a quad or a jeep wouldn't get me any farther than I can get now, so none of those are viable options.