Before I had a dog I could not stand dogs on the trail, actually now that I have one I still get pissed by most of the unleashed (and leashed on single track) dogs I encounter. Before I got this dog I would have said I would never ride with a dog, leashed or not. Now I ride with her quite often - but there are a few unique things about her, and I use large dose of common sense before I take her with me.
Her unique qualities:
She likes me and my family, and a couple of my kids friends - and that is it. She does not like anyone else, she will not approach them - and leaves plenty of room to pass, usually she gets off the trail to let others pass.
She is afraid of most things other than ripping single track, when we see a deer, moose, etc. she goes the other direction.
She does not bark - really (only at strangers approaching the house) She has never barked on the trail.
She will approach other dogs but she is scarred, and will turn tail if they growl or bark.
My common sense:
I only ride one trail with her - it is mainly a hiking trail, ~10 miles with about 2000' vert - I ride it a couple hours before sundown and if it is under 75f (which is often around here). Most people are off the trail - about 60% of the time we pass no hikers, and 80% no riders.
I carry a leash - just in case, but have never had to use it.
I did bring her with on a ride with a freind once, but I felt that was rude of me (he thought she was awesome though) so now I ride with her solo or with her and my kid.
If she craps - I kick it off the trail - I hate poop on the trail too.

Am I breaking a law - technicaly, yes. Am I inconveniencing anyone, or creating a potential for disaster? Nope.