adding the components for a removeable battery to be possible doesnt add "That" much extra bulk, look at how small some phones are out there now, and every single phone to date has had a removeable battery (well every phone i have ever heard of, touched, looked at, etc has had a removeable battery). also its a security and safety issue, having power when you really need it is essential in emergencies (car accident, people drowning etc, etc)
most phones even have a choice of battery capacity,
for EX: both my HTC Prophet and TyTN have the standard battery as well as doublesized batteries. the doublesized batteries come with their own battery cover, ok, so its makes the phone bulky, but you have litterally 2x talk time, instead of 5 hours you have 10 hours. the phone will fit comfortably in my pocket with either battery so being thick or thin is only an issue if it creates discomfort
well actaully you have 15 because you have the original battery too. cold swapping the battery takes 30-60 seconds, maybe even quicker depending on the phone.