After years of using a PC Tower sitting on the ground, I got tired of climbing under the desk each time I needed to unplug the ethernet or disconnect the monitor.
Switching to an all in one computer was one of the best thing I ever did. It makes growing older so much easier. At the same time you learn to select the specifications that you want and live with it for a few years, not having to get in there and fiddle with drives and wires etc, sometimes you just want something that just works. Get in do what you need and get out.
Something like the HP all in one computer shown here is also ultra portable since it is made of lightweight polycarbonate housing. Something like this works well for the gamers who would like to attend LAN parties and not worry about all the cables and monitors etc.
The Wired article listed above goes into much more discussions as well as the pros and cons of an all in one PC.