I use harnesses and leashes. I do rabbit hopping, so need them for that as well.
I think that they do have uses, but you need to take into account the rabbit and where you will be using it. Not all rabbits will accept the harness and it is a good idea to work with them on wearing the harness before taking them out.
You do need a suitable rabbit harness. H style ones are best. Avoid figure 8 ones as they can tighten on the rabbit which could choke them. Vest ones are ok, but are bulky which can restrict the movement of the rabbit and they seem easier to chew. The velcro is also not so good as fur can get stuck in it, so make sure there are buckles on both the neck and chest. Fit is also important, too loose and the rabbit can get out of it, too tight and it can be uncomfortable or restrict breathing.
As for leashes, you don't want to too short or the rabbit can't go anywhere, but too long means they can get tangled. I would not use a retractable leash because if the rabbit runs to the end, they can get jerked back. Retractable leashes also keep tension on the harness. You want a light leash with a light clasp. A small cat or dog leash will work. Most small leashes are 4ft, but you could connect 2 to make it longer.
I would avoid using a harness in an area that is not rabbit safe. You want to make sure that the area is ok for a rabbit as they could get scared and bolt which could be dangerous if you are not paying attention. Don't take you rabbit to a place where there would be lots of dog or something.