Why? If your carry-on is within published guidelines, they won't bother you (if they do then yes, get the supervisor and put the TA/GA in his/her place). If bag is too big or heavy, the TA/GA has every right to request it be checked - it's part of the TA/GA's job for everyone's safety.

Certainly, if it's just a little over, the staff should exercise discretion and not let a minor thing like a couple of pounds over the limit upset a customer. However, if it is way over, they should absolutely insist the bag be checked. Just search in FT (don't even bother with google) for incidents of overhead luggage falling out and injuring passengers and/or FAs.