If you travel, it's one more thing to carry. Plus, I occasionally forget to plug the phone in to charge at night. My current smartphone makes it through two days without charging, though.
That's a good point. You'd probably want to carry the charger anyway (unless it's just an overnight trip and your phone lasts many days) but even just the travel itself can put extra demands on the phone, because of time zone changes or watching lots of videos or whatever. A phone that's perfectly acceptable because it can last 18 hours in normal use can cause problems if you're taking a 14-hour flight and playing with it the whole way.
