Working in a restaurant located in a business hub, I see two or three blue-tooth headsets each day. Sure, maybe not a lot by your standards, but it's significant that these are the people who forget that they're wearing them meaning many more probably use them regularly.
I use one, myself, because I'm never just talking on the phone. If I'm driving and a phone call comes in, I can't be holding the phone to my head. I don't often play games that can be paused so I can't stop just to answer the phone at the drop of a hat. While I'm walking around not doing anything else, sure, I could probably just hold the phone up to my head, but I already have a bluetooth earpiece. Why would I not want to use it?
On the topic of how often any of these come up, very rarely. My last phone bill had 6 minutes of airtime used for the month. Still, I've had the earpiece for over a year and the convenience for the above-listed situations makes it more than worth the $40 I paid for it.