I love dogs and have mountain biked with my old dog (an aussie), but it really is tough on them. Dogs really can't hang with a fast mountain biker over 8-10 miles, although they'll certainly risk death trying. I think they'd be happier if you were on foot and just hiking with them off leash.
Also, its pretty inconsiderate of other riders. I've gotten tangled up in happy/friendly dogs chasing me a number of times. Could get the dog or someone hurt.