They are not agains the rules, they are against the social aspect of the game. If a person doesn't help look for a disc they are in violation of the rules, and should be called on the infraction. If they are not paying attention and it is distracting to other players, that becomes a courtesy violation.
Speaking of Deaf people, I was fortunate enough to play a round with 2 deaf people and one person wearing headphones in Iowa City for an event....the lonliest round ever......
Some people should be given courtesy violations for being too talkative as well, you simply warn them, and say, look I will not give a courtesy yet, but if you don't refrain from talking I will next time.
That typically will shut them up.
Hurting our image....give me a break, headphones are not hurting the image, its all about the attitudes that people play with and the desire to show how unprofessional we are that hurts our image, not headphones.....
I am also with Ash and Drew though the need to carry about with a set of external speakers is not something that I personally think is ok, it doesn't bother me at casual play, but I think that leagues it should be asked ( I usually don't really care, but would prefer that we not have a jam band going on during league) by the person porting in the sound for the group. I have never said no, and generally I am asked, so if you are that guy, just ask.
During PDGA events, or even club tourney's, I would find exception to it, unless its something completely for fun.....