In preparing for my recent Half Dome hike, I encountered many commenters on bulletin boards saying it is unnecessary and even dangerous to use a harness on the cables. I have to strenuously disagree and advise anyone who intends to ascend the cables acquire gear for clipping in and familiarize themselves with that gear. While the cables can be climbed without any safety gear, I found it greatly comforting to have the backup of a solid connection to the cables.
I have been climbing for several years, and am comfortable with a harness, clipping, and basic anchor principles. The gear I chose was my climbing harness, a pair of 24inch dyneema slings girth-hitched to my belay loop, and a pair of carabiners to connect the slings to the cables. As I ascended, I kept one biner clipped to each cable. When I reached the next rung, I unclipped and clipped each biner. If someone had to pass or when I descended, I clipped both to one side. In this way, I was never without an anchor point.
I want to counter some of the various arguments I have seen on the bulletin boards about using a harness on the cables.
1) It is too slow and you'll upset people: First of all, when engaging in a life-threatening activity, safety should always take precedence over convenience. Next, the traffic on the cables is such that I did not feel any slower than the unclipped people ahead of me, especially being familiar with my equipment. Also, I personally felt that having the safety-net of the harness allowed me to be more comfortable and move more quickly. I can tell others felt similarly: one young woman I saw on the way up was clearly scared, but moved slowly and consistently with her harness. Another woman I saw on way down was not harnessed, and became terrified and panicked, endangering herself and others. She caused a half-hour backup on the cables. So who was really more inconvenient?
2) The cables are not designed for gear and your gear may break and prove useless: Climbing gear, even static webbing and carabiners, is designed to withstand much greater forces than would be present in a standing fall. It's true that a dynamic via ferrata cable would have been ideal, and it may be possible that a long, fast slide resulting in the contact of carabiner and pole could break a biner. However, it is hard to imagine how being clipped in could make the situation of losing your grip and sliding quickly worse.
3) You could slide down the cable, knocking others down, whereas without gear you would fall to your death all by yourself: This is ludicrous. Nobody that I saw stands between the poles. It is very uncomfortable to stand without the planks for support. Thus, if you fell clipped in to the next pole, you might bump one person.
4) You could become distracted with the gear and do something more dangerous: This is a fair point. That's why anyone intending to use climbing equipment should take it upon themselves to be competent and practiced with that equipment. I saw too many people who bought a quickdraw at the mountain shop and clipped it to their backpack and thought they were safe.
5) The cables can be ascended without gear so you shouldn't use gear: This is a fine thing to say from the ground, but I can tell you that nearly everyone I passed on the way down wished they were clipped in, and said so. Competently using safety gear adds a level of comfort to the experience that cannot be described in words. But if you are the kind of person that wants to do something the more dangerous way just to say you did, go right ahead.