I prefer using a device that isn't my phone to listen to music, because my iPod doesn't pester me with notifications, calls, emails, social media, and so on and so forth. There are less distractions, and you can focus on the music more easily. I also believe in decentralization as a means to improve my experience with everything over all. I would way rather lug around a backpack with a good device for every task - a camera that is a good camera, a laptop to type on, and a gaming handheld - than one device that is mediocre at everything, like a phone that has a mediocre camera, a keyboard that will give you carpal tunnel syndrome faster than you can type the words "carpal tunnel syndrome" on it, terrible touchscreen controls that make for a horrible gaming experience, and awful locked-down software that steals your data and kills cute puppies or something like that.
Also, it reduces my dependence on my smartphone. Talk about putting all of your eggs into one basket. If you use your phone for everything, and your phone breaks for some reason, everything is gone. If you have a dedicated device for most tasks, your phone can break, and your sentimental photos of christmas 2012 are still on the SD card in your camera. Your songs are still on your iPod. Your savegames are still on your Nintendo whatever-it-is.
The longevity is improved too. My Canon DSLR from circa 2010 served my dad well, and now it serves me well. I have had no problems with it.
Same goes for my iPod from circa 2009. It served my dad well, and now it serves me well. I have had no problems with it.
Same goes for my Nintendo DS from whenever the Nintendo DS released. It served my dad well, and now it serves me well. I have had no problems with it.
I challenge you to find someone who still uses a phone from 2009. The old cell standards are unsupported, the software and operating systems don't get updates anymore, and so on and so forth.
Hope this helps.