They're not neccessarily dangerous, though I wouldn't recommend them in city traffic.
I use them specifically for city traffic, but that's just
my crazy self. Also country lanes. Everywhere, really…
It's curious what makes some of us comfortable with them, others not so much. I can appreciate people who have no place for them while cycling, whether it's because of concerns for safety or they prefer listening to the real life soundtrack of their ride. For my own part I've been plugging in since my first end-to-end back in 1997, which if memory serves was accompanied by an awful lot of
Tanita Tikaram on cassette tape. These days I'm thankful for mp3 players.
In the city in particular, music relaxes me (I'm sure it would have the opposite effect on others!) It filters out much of what I don't need to hear anyway, while still letting in things like sirens.
In the country, frankly, it gets my mind off how boring the country is, much as I love it here. It also provides the occasional surreal moment such as I experienced yesterday evening as I glided past a well known grazing animal and this came on:
View: https://www.youtube.com/watch?v=khs_PofcsbQ
Actually it was a cow I went past. I like cows, and not just because Gary Larson did so much to make them interesting. They have big beautiful eyes which follow you as you go past. I often wonder what they're thinking, and if it involves regrets at not being a carnivore.
A lot of people think they're dangerous.
Headphones, I mean. They are quick to give me and others like me Darwin Award points. I just show them my organ donor card and carry on.