Closing off the back side of the speakers to the outside necessarily requires acoustical compromises that guarantee uneven frequency response, which is why open-back headphones are preferred when greater accuracy is needed. They also tend to be at the upper end of the price spectrum, not because open-back headphones are more expensive to manufacture, but because they assume that consumers who buy them are a) more discerning of quality and b) willing to pay more for it. They tend to have little details like metal hinges and detachable cables that add to the cost.
The most popular headphones you see people walking around with are definitely not high-fidelity transducers suitable for critical listening. But they aren't
just fashion statements, either. The appeal, I think, is the improved bass response and greater isolation compared to stock earbuds. The Beats by Dre class of headphones are bass-hyped, making them unreliable as primary studio references. But they'd probably be useful as secondary references, to hear what your mix sounds like on that type of headphone.
For myself, I am very happy to see the trend toward higher-fidelity headphones among young people. It might not be the listening experience I'd prefer for myself, but it's a huge step up from those crappy plastic earbuds that come with iPods! Many become accustomed to highER quality sound, and then start questioning low-bitrate MP3s. I'm surprised at how many teenagers actually know about the existence of the FLAC alternative now.
Everybody here can do one thing to help save the next generation from sonic mediocrity: make a point of sitting at least one teenager down and making them put on your best studio headphones for a listen. Watch their eyes widen. You've just broadened their world view! Then hide your headphones so they don't steal them.