I actually find it useful. Needed a preceptor for my clinical rotation and found someone on LinkedIn. So it had its uses for me in the nurse practitioner field. Would guess it would be good for anyone in health care. Have had a few recruiters contact me about future jobs.
Essential if your in a finance or consulting capacity. I used to think it was a waste of time also, but have truly found it to be a great tool in networking. Finance and consulting are entirely driven through networking to find work.