On the other hand, from a mechanical engineering stand point, when you power up a computer, it heats up. As things heat, they expand. When you shut the system down, it cools, and contracts. Things only expand and contract so many times before they fatigue and break. While we're talking about a low amount of fatigue (after all, the temperature differential across most of the system isn't all that high), we're also talking about a relatively tiny component.
Now I haven't studied this, keep in mind... but you might want to take a look at these:
Google Search: power computer off versus leave it on