I used to use 2 batteries on an older Toshiba laptop, but I don't anymore. My iBook had such long battery life, I didn't even consider getting a 2nd battery at the time. (and after 3 yrs the battery is still in very good shape).
My MBP doesn't last as long though, so maybe one day...
If I may suggest something to Apple: make the top cover of the MBP a solar panel, this way we can just lay it out in the sun to charge and I don't need to haul around that large charger that comes with the new notebooks
(would work better on a ultra-low-power-flash-drive-13" though)