I've seen so many people experience a "dead battery situation." Maybe they forgot to charge their smartphone last night. Perhaps their phone was frozen and it sucked all the juice out of the battery. Maybe you had to take call in the hospital at the last minute and you didn't have your phone charger. Many of us would admit that we'd feel utterly lost without our smartphones. Given that our entire lives are on these devices, we really can't afford to experience a dead battery situation.
For those of us who have a user-removable battery, it's easy to keep a spare in our bags. Dead battery? No problem! Pop out the old one and insert the new one. Perhaps you even have the luxury of using an extended battery.
For others who have smartphones like the Apple iPhone, you'll need to keep an external battery pack (who else makes smartphones like this?). I've heard rumors that the Apple iPhone 4G will have a user-removable battery. Let's hope that this particular rumor is true. Would you be willing to buy an iPhone that's slightly thicker if you could remove the battery? Given that the lifespan of a typical lithium ion battery is roughly 2-3 years (before you see significant decreases in performance), I think that iPhone users will want the option of changing the battery themselves.