Some folks I've run across these past few years have been reticent to let other people use their computers. Along with wondering exactly what they are hiding on their desktop, this decision has always dismayed me. If this was the late 60s, there would be a movement towards open-source computer sharing. Bring your laptop to the quad! Free wireless, baby!
But one of the best reasons I've found to let other people use your computers is that they can give you hints on how to best optimize your experience. The other day I let my friend Joey use my computer and he showed me the useful hack about how to best organize my bookmarks on firefox, which has allowed me to waste my time on shelfari and imdb more efficiently. So, let other people use your computer.