I always prefer positive training methods and NILF training. I wouldn't ever feel comfortable using them on a client. I have never needed one for my dogs, as they are pretty well trained.
That being said, in the hands of somebody who is trained in their usage, using the right collar, and uses the lowest possible "pressure" (shock level) I think they are ok as a last resort. It's really a slippery slope. One person could over use the shock feature out of frustration (or inexperience), while another could improve the quality of life for that dog (think: being able to have a dog trained on a shock collar so they can be off leash, whereas the dog would bolt and never come back before)
IF I hypothetically were to use a shock collar (never with any dog that wasn't my own) it would be a last resort, to protect the dog from something worse than a small static shock. (Like the dog getting hit by a car from digging out of the yard or jumping the fence) I never plan to train with a shock collar, but it's comes down to a matter of handler experience, personal preference, the unwanted behaviour,and the individual dog. Most behaviours can be fixed or prevented with exercise and positive reinforcement training. Some dog breeds are prone to certain behaviour problems, so researching a breed before you buy is a good step when becoming a dog owner :)