Along with the Grandfather Clause on Courtesy....
Some people think that Headphones are a replacement for Helmets.....
I love music...
Music can get us pumped.
Cruising on a snowboard or doing along road ride can be enhanced by the right soundtrack....
with that said....
I opt out of that option. We only have five senses, and hearing is greatly tied into feeling. If we have music blasting in our ears we will not hear that Caprice Classic with paper tags rolling up fast on our asses....no horn....just the implied....."move or else"...I have always moved, never tried to learn what the "or else" means, never flipped off this driver always let them pass on their way.
Headphones will also take the rider out of the moment and take them to a different place...
that place may be in the clouds, while actually the rider should be here on earth in the moment
living it
feeling it
watching out for other riders, obstacles, and the unexpected

this is not to say that I do not occassionally bend these rules
on long tours I may need to break it up a bit
or if I am alone and only riding cruisers on my snowboard
for me very few rules are entirely set in stone
but the routine of the headphones is a dangerous one
play the odds
odds are that a person with headphones on is more likely to put themselves in danger
than a person using all of their senses to take in their surrounds

and this totally ignores the notion of all the wonderful things to hear on a bicycle ride
including silence