Impress your friends.
Develop and maintain your fine motor skills.
Your CPU has only a finite number of cycles before it becomes senile. Don't waste them running Windows 95.
Librettos are the environmentally conscious choice for running X11 -- they conserve endangered pixels.
Unlike other notebooks, a Libretto will fit in your pocket -- if you have large enough pockets. (Joking aside, mine gets a lot of use because it goes everywhere; unlike other laptops, there isn't a major ``threshold of need'' that must be exceeded to outweigh the inconvenience of dragging it along.)
With xephem (see
http://iraf.noao.edu/~ecdowney/xephem.html), a Libretto running
Linux is smaller and more useful than a set of star charts for amateur
astronomers. If you aren't an amateur astronomer yet, get xephem and
look up at the sky tonight.
Do anything your workstation could do two years ago, anywhere.
Compile new kernels while commuting. (Demonstration performed on closed roads by professional drivers. Always use safety belts. Kids, don't try this.)
If the One True Editor was meant to run under Windows, it would have a menu bar instead of key bindings like Esc+Meta+Alt+Ctrl+Shift. (What's that? It has a menu bar, too? Never mind, then....)
Who needs floppy disks anyway?