Doing work from customer sites, where it's not always feasible to plug in. And driving from site to site, where plugging in isn't even possible.
While it is true that I have seen many people who buy a laptop just to use it as a small desktop computer; there are those of us that use the battery all the time. I bought a 12" PowerBook G4 in 2003, and have two batteries. Between the two batteries, they have a total of 500 charge/discharge cycles. I bought my 15" MacBook Pro the day it became available, (delivered in March 2006,) and my original battery had 50 charge/discharge cycles before I returned it under the warranty, and my two replacement batteries have a combined total of 180 charge/discharge cycles on them, in less than one year. (When I got the warranty replacement, I bought a second one at the same time.)
Just because *YOU* don't care about battery life doesn't mean that NOBODY cares about battery life. I, for one, would love to have an extra hour of life. I completely discharge one battery during the day at least 3 times a week, and sometimes completely discharge both batteries before I get an opportunity to plug it back in. Not "before I feel like" plugging it back in, but "before I get an opportunity" to plug it back in. I have had a few days in the past year that I really wish I had a third battery. (Since I could get 5.5-6 hours out of my old PowerBook when I really skimped, and I can only get 4.5-5 hours out of the MacBook Pro.)