while road riding the ability to hear what is around you is part of feeling what is around you...that is really two senses... not one
cranking music directly in your ears will not only remove the ability to fully take advantage of those senses
but the music in your ears will also rob you of your focus
while mountain biking the dangers will vary...
sure there is not the fear of a soccer mom wiping her skirt clean because she spilt her coffee while checking her blackberry
but...
there will be cyclist trying to pass you as well as cyclist heading your way around that blind curve
seconds could be the difference between a clean pass and locking handlebars thus locking horns
in addition to crashing into another cyclist, a hiker, or any other life on the trail
there are the sounds of the bike
that clicking of a derailer mal adjusted, pulling slowly into the spokes as the limit screws are poorly set....
the first two clicks are warnings
the next sound will be your deraileur being drawn into your rear wheel killing your wheel and your deraileur hanger
or a stick in your wheel....
the hiss of a slow leak...
the popping of the chain...
whatever
the bike makes noise
much of the noise is a good warning
listen to the bike while on the trail
listen to the music when headed to and from the trail
or better yet
put the head phones on and stay on the trainer in your basement