In my view, you are sort of assigning value to the "work" being done by desiring the bitcoins in the first place... I see it as saying "I want to use these bitcoins to purchase a good or service (or contribute to person or cause) because they are of value to me, by virtue of the fact that I expended a considerable amount of time and electricity to generate them."

I do see your point about the computations themselves not contributing to the greater good in an obvious way, but you could always consider it as a contribution to society in the form of a scarce, secure, anonymous currency.

Or, if it still bothers you, you can always find a good (honest) method by which to gain large quantities of bitcoins, and use them to pay others for running Folding@home, SETI@home, etc. instead of generating coins / transaction fees! In face, if you wanted to pay me a set amount of BTC per F@H point, I'll switch all my cores over right away...