The has been quite a ruckus recently in the Ann Arbor News concerning unleashed dogs in local parks. Some have characterized such canines as the Packhounds of the Apocalypse, or as the direct descendants of the rabid hounddog from To Kill a Mockingbird. Part of this fear is based on experience with dogs with slob owners. If your dog is aggressive, you can't have him off a leash, period. You should clean up after your dog, although the claim that a few dozen dogvistors will choke the local park watershed with poop is far fetched (if not, we had better also round up and cage the permanent park residents like squirrels, raccoons, birds, and the feral cats).
But it's cruel to keep a dog penned, housebound, or leashed for the entire lives. Dogs need to sniff, explore, woof, point, chase things and dig stuff up - much like guys. Unfortunately it is possible in early 21st century OprahAmerica to deny these things to dogs by the compulsive "I must have complete access and total control over my environment on my terms and at my whim" types, assisted by hyperlitigous bottom feeders and spineless bureaucrats. Therefore, we need some natural areas that are off limits to such control freaks but are reserved for dogs and their human companions to explore their universe.