Te-wa - I'd pitty the person at the other end of YOUR leash. Talk about a workout!
I frequently hike with my dog off leash. Never in areas where dogs are prohibited, mind you, and my dog is very well behaved, too timid to really ever want to get close to other people or dogs, let alone bite them, and she's generally frightened of wildlife as well (you should'a seen her the time she accidentally flushed the flock of wild turkeys! Classic!). Now, I totally think that there are problems out there with other dogs, with people who don't like dogs, and with pets having a negative effect on wildlife, etc. However - there are also conflicts between people on mountain bikes and hikers, equestrians and mountain bikers, humans and wildlife, etc. Being out on the trail means being responsible for yourself and your livestock/pets/equipment. All of these farking rules exist because you can't trust people to be well behaved and to have adequate control over their charges.
BTW - living in Tucson and wanting to go hiking with your dog completely stinks. Saguaro National Park (both east and west districts), the Pusch Ridge Wilderness and Tumamoc Hill are all off limits to dogs, period. That means 90% of the trails closest to my house are no goes if I want to bring my very favorite hiking companion along. Especially given that it's generally accepted that the bighorn sheep population on Pusch Ridge is no longer viable (if it exists at all) and the fact that many of the SNP trails are so remote and untraveled, it seems unnecssarily restrictive. I'd even go for hiking with her on a leash if I could take her to some of these wonderful hikes...