I had a colleague who insisted on giving a flight attendant a $5.00 gratuity on a two hour flight (in FC), I was mortified, I cringe thinking about it, I believe the flight attendant was offended, I know I would have been if I was a dedicated professional.

During grad school I had an internship in a professional position where I occasionally came into contact with spouses of VIPs, I was always very gracious and helpful, I made suggestions for restaurants, getting around, things to do while the spouse was in a meeting, etc. Luckily nobody ever offered me a gratuity but if they had I too would have been offended.

The way I see it, professionals may be gracious, and may be service-oriented, but they aren't providing a service as a function of their core responsibility. Flight attendants are safety professionals who enjoy providing a pleasant experience for their passengers, they aren't hotel concierges whose primary purpose is to provide guest service.