...
Finally, it's not like the computing power that goes to mining is general purpose. It's a very, very specific math problem that is most efficiently done by highly specialized processors with chips (ASICS) built to mine Bitcoins and nothing else. You just can't use that computing power for anything else, and if you could, Bitcoin wouldn't be secure because you need that math problem to have very specific properties, properties that mean it has to be useless for any other purpose.
Actually, you can do merged-mining for bitcoins and other alt-coins (not Litecoin which uses scrypt) and have Bitcoin be just as secure. Also, not everyone uses the same equipment, with the same dedication, for the same reasons. For example, it's still quite possible to mine bitcoins with a standard desktop computer. It's probably not very smart or efficient, but it's possible. Some people may mine for novelty or curiosity or fad or whatever else. Not everyone will use the most specialized and efficient methods, but every computation that goes into mining, at any level, adds that much more security.