Agreed. The case for having a case is when you want to keep resale value as high as possible, but even now that's not such an issue if you do the upgrade program where you automatically get a new phone every year and basically lease it.
What are you doing where it's unusable with a single charge? Most days I get by fine or mostly fine with my phone and I'm a pretty heavy user (music, web browsing, fb/twitter/instagram).
I had to get an external usb battery but only use that when I know I'll be busy a full day with more than usual usage (which is still quite a bit). Example being a full day of work (commute in and out) plus a sportsball game at night or similar where I'm seriously out all day.
