When I travel I always bring along more than one pair of headphones. The reason used to be so that if I lost the first pair, as I did once in an Airport in Atlanta, I have spare one. That saves me from the temptation to buy a replacement pair at the airport, which is not the most cost-effective location if you're looking for a deal...
So, if I don't carry a second pair of earphones as a back-up, why do I bother with the extra weight and bulk? Simple, Sometimes I want maximum isolation, and sometimes I don't.
Basically, there are three varieties of earphones; in-ear, on-ear, and over-ear. Each design has certain advantages. In-ear types usually deliver the best passive isolation because they provide the best seal and the most blockage from outside noise. When I'm actually IN an airplane I reach for either my Etymotic ER-4P or Ultimate Ears In Ear Reference Monitors, both of which deliver greater than 30 dB isolation. I don't care about screaming babies or even three-year olds in the row behind me - once the music is playing those minor distractions are gone, as if a big wind just sucked them out the rear exit doors. Nice.
But sometimes isolation is not a good thing. Two weeks ago when I traveled down to Dallas from Denver via American Airlines, on the return flight American changed the gate three times in less than an hour. Obviously, if I hadn't heard the notifications about the changes I might still be sitting in the wrong waiting area in Dallas...but since I wear a different, less isolating pair of earphones when I'm in the terminal I could hear the flight changes, so I made it home on time, on the right flight.
An earphone's ability to isolate you from your environment can vary from "none," such as with open-back headphone designs from Stax and Audeze, to "some" with closed-back on-ear designs from B&W and V-Moda, to "lots" with in-ear monitors from Etymotic and Ultimate Ears. Also there are two varieties of isolation; incoming and outgoing. Sometimes, such as on an airplane, you want maximum isolation for both incoming and outgoing sound. But other times, like in the airport, the ideal is only partial isolation from incoming sound while preventing your music from disturbing others. Headphones such as the V-Moda M-80 or the Beyerdynamic DT-660 provide just this kind of isolation so you can hear the outside world, but it can't hear you. In public environments, such as offices or transportation hubs where you need to know what is going on around you, a semi-isolating earphone is the best option.
And what about open-backed low-isolation earphones? I leave them at home, where they belong. Almost all of the best-sounding, and most revealing earphones I own are open-backed non-isolating designs such as the Stax SR-5 and Beyerdynamic DT-880, but to appreciate their qualities you need to use them in a quiet environment, so these headphones never leave my office. Instead, while on the road they provide another (as if I need one more) reason to return home.
So, now you know why I travel with more than one pair of earphones. I'm a firm believer in using the right tool for the job, and for travel the right tools are more than one pair of headphones...