It's convenient as long as you have access to it. I put my entire music collection in FLAC on a microSD card and can listen to it when I walk my dog without worrying about if my phone has enough charge, data rates, cell phone coverage or any other bullshit because I have dedicated media player.
Google Play (another $10/month on-demand add-free music streaming service) lets you download albums via wifi for playing later, so you don't need to worry about data rates or cell phone coverage. I would assume other similar services allow the same. Admittedly you're still SOL if you can't manage to keep your cell phone charged.