Batteries are replaceable, just not by you. Do some people replace their phone when the battery sucks? Sure, maybe. But then that phone probably gets refurbished for someone else to buy.

The main use of user replaceable batteries for me was always when my phone only lasted a few hours so I had to swap another one. My Samsung Blackjack with 3G would die in like an hour if I was actively using the 3g connection lol. G1 and HTC Evo also didn't last a whole shift at work.