Even if you play your music at a low volume, your ears were not meant to be covered by ear buds or over-ear headphones constantly (as in, hours at a time). And yet, this is something that people in the IT industry have accepted as common practice. Just think of the looming health implications when everyone's ears get a little older, and less likely to heal properly.
About 5 years ago I had an ear infection that left one of my ears with a permanent loss of hearing. Now, it's only 5% at the top end of the spectrum (soft Ss, iirc), but it's enough to aggravate me daily. But the downside is that I can no longer wear headphones for anything more than 20 minutes at a time (it feels like pressure is building up in my head...a very weird feeling), even with no music playing. So here I would sit, in a loud open plan office without the ability to wear headphones and being asked to solve hard problems. If this were a leg or back injury and I was made to use something in the office that aggravated that problem area, I'd have a worksafe (OSHA?) problem, but for some reason people's ears are not a health issue.
Just remember, your ears are extremely sensitive instruments and they rarely get better if you destroy them. And if you think not being able to hear properly is the only side-effect, just ask someone with hearing loss what they go through. They'll likely tell you about slurred speech[2], random phantom noises, pain, etc.
[1] http://www.osteopathic.org/osteopathic-health/about-your-hea...
[2] This is caused (over time) by you not being able to accurately hear what you are saying. You hear your own voice via two mechanisms: internally through your jaw bone (mandible) and externally through your ear. Fun fact: most of your voice is heard through your jaw, which is why everyone says their voice sounds funny through when they hear themselves on video.
[Edit] spelling