Simply put, no. Rabbits don't NEED UVB rays. But they can definitely benefit from exposure to them.
If your rabbit lives indoors, consider investing in a fluorescent fixture and a UVB emitting fluorescent bulb. These can be purchased from the reptile section of any pet shop and placed about 12-18" above your rabbit's cage or area where he spends a lot of his time. If your rabbit lives outdoors, like in a hutch, he is already getting UVB rays from the sunlight.
Mercury vapor bulbs are alright for large spaces where your rabbit roams (like a bedroom), but they also emit a great deal of heat, so unless your bunny likes it toasty, these probably the best choice for rabbits.
Studies have shown that rabbits (and many other pocket pets) have an increase in activity, appetite and overall happiness with the addition of UVB rays. Many rabbit owners find UVB rays especially useful in rabbits who are sick or recovering from an illness (such as ileus), new and stressed out rabbits, and rabbits who have recently lost their cage mate.
Just be sure your UVB light isn't too close to your rabbit or he can get sunburn or hurt his eyes. Keep the light on for no more than 12 hours a day, and replace the bulb every 6 months, even if the light is still working since the invisible UVB rays run out after about 6 months, depending on the manufacturer.
