Primarily the main reason for wanting such extended battery life is to keep your battery running long enough to make it through a long flight without needing to recharge. Almost every other situation in which I could require extended battery life puts me in close proximity to a plug or car-lighter where I could plug in.
On a sidenote I have been quite happy with the laptop-mode patch that exists in kernel 2.4.24 (i know it is in some previous versions as well). It makes a lot of sense because it keeps the kernel from making writes to the disk too often when there shouldnt be a whole lot of activity going on anyway. Things that also helped included switching to metalog (a buffered logger) away from syslog-ng, and installing CpuDYN to do adaptive cpu throttling. These solutions are awesome if i stick to running pure linux applications. However , opening up a VMWare session pretty much throws all power management out of the window, as you cant count on the child OS to behave.