I use a harness for agility and when I take my rabbits out. It is mostly for safety and to have something to hold onto. I don't really use a leash that often though.
When it comes to harnesses, some are good and some are dangerous. Any harness that is all one piece (figure 8 types) are dangerous as they can tighten and choke a rabbit. Vest harnesses can be hard to adjust to get them to fit right and they are bulky, but otherwise are pretty safe. H style ones are safe to use and can be adjusted to fit a rabbit fairly easily. Shoulder harnesses also seem pretty safe.
Leashes do need to be used properly as they can be dangerous if not used right. There should be some slack in the leash so you are not pulling the rabbit around. Since rabbits tend to stop and go quickly and change direction with no warning, the person holding the leash needs to pay attention to avoid the rabbit getting jerked back with the leash.
I think that if used properly and safely, harnesses and leashes can be good. They should not be left on unsupervised.