How common is it for someone to need a lumbar sympathetic block?

Not very, may need.. The sympathetic nerves coming out of the lower spine, lumbar, send pain impulses down the legs when overstimulated in problems like herniated lumbar discs, severe degenerative arthritis, tumors pressing on lower spinal cord... If pain or arthritic meds, electrical stimulation devices (implantable), epidural injections, morphine pumps all don't control pain, may need to destroy pain nerves to legs.
Not very. A lumbar sympathetic block is used to diagnose and treat sympathetically-mediated lower extremity pain. This condition occurs relatively infrequently but can be devastating if it's not caught early. If your physician suspects that their may be a sympathetic component to your pain symptoms, a lumbar sympathetic block would help make the diagnosis and hopefully provide some relief.