My grandma had a cat who was leashed when he was outdoors. She did not take him for walks though. I really wish she had because he could use the exercise, but they didn't really think about that I guess. They had a dog as well and so they would put both of them on leashes which were clipped to a long taught line that went across the entire back yard so they could run around in the yard but were unable to get out of it. They did this because they were concerned that the cat, who was 3/4 Siamese, would be stolen if he was allowed to roam outdoors.
I've seen a few people in my town walk their cat and dog together and they don't seem to have any trouble with it. Initially it did look odd to me because cats are usually so free spirited that leashing one seems to be very contrary to their nature, but since none of the cats I have seen walking on a leash seemed unhappy, I think it is fine to do it you don't want to let your cat outside to roam.
