It depends entirely on the situation, the setting, the dog and the owner. My own dog stays immediately at my side even off-lead, so I tend not to use one. However, if I am at a state park that has a leash law, in a situation that has other dogs off-lead, or in a noisy and crowded setting that may confuse my own dog, then I use a lead. Unfortunately, many owners who allow their dogs to roam off-lead do not have adequate control over their pets. And, no, using a leash on a dog is not cruel.