I fly so much for work that I rarely fly for leisure. Mr. Kgsd can never understand why I don't want to use my miles to take trips on vacation. Actually, I end up giving much of my miles/tickets away.
My first travel job was for a telecom company - I travelled to client sites for new installations as project manager and to train the client's staff on our system. Most of our clients were hotels, which meant that since I usually stayed at the client hotel for free (as part of my company's contract with the hotel) I didn't get any frequent guest points. Although they were usually very nice hotels.
My current job is in the employee benefits/actuarial field, and I visit (existing) clients to explain things to employees, on-site administrators, and board members.