I think when you use XP with centrafuse or RR as shell, you pretty much have it all.
1. It's quick and easy to install, and almost always installs without a hitch
2. It only takes about an hour to update, disable services, and perform speed tweaks, then you're ready to install software
3. Software. Theres lots of it, most of it is free or can be had for free quickly, and it's well supported because theres a huge user base. Theres hardly ever a problem that a quick search here or google can't fix in a hurry.
4. You can use S3 standby and have immediate up/down. Worst case scenario you need a new battery every couple years... but probably not.
5. I've had some customers using XP that always standby or hibernate and haven't rebooted the system in several months. It always just works.
6. Hardware compatibility. 99% of the hardware on the shelves will be compatible with XP. You don't even really need to check. If you're mac or linux, it might work, it might not. You might have to customize a driver or other such dicking around.
7. Familiarity. Everyone that jumps in your car sees windows XP and immediately knows how to use it, even street bums know how to use XP. By contrast, When my GF sees a linux desktop, even though its similar, she doesnt have a clue what to do and wont even touch it.
I could go on.. but I think it's pretty obvious that right now XP is the logical choice. I've seen some nice mac mini installs, but I find them quite useless without a fully developed frontend and good GPS program. Its like back in the day when we were trying to use winamp with touchscreens..