This is a slide from a report I wrote over three and a half years ago on Mobile Music Phones.

In other words, the more successful a phone is with its design, the more people want to use their phones various features, and so the more strain is placed on the battery and a phone's daily life expectancy falls.

Related research reports:
Mobile Devices – Meeting the Service Needs of Upgraders, published on August 16, 2008.
Mobile Music Phones, published on January 12, 2005.