I also have a portable battery charger- great for taking away over weekends, several phone charges worth, with 2 USB output sockets. Making me free of having to buy spare batteries that are out of date when I get a new phone (http://www.techradar.com/news/portable-devices/other-devices/best-portable-chargers-14-we-recommend-1178873#articleContent)
Yes there is always the thought that my inbuilt phone batteries will die, but I haven't had that problem (yet)- I had an iPhone 4S which lasted over 2 years (went to my wife after a year). The battery didn't need replacing, but it was easy enough to do. I've never kept a phone for 2 years, and whether a phone has a replaceable battery age didn't make for a higher selling price so it's a mute point.
Now if only batteries would last a few weeks.