Who cares if what we do is perceived as being useful. "Useful" is a matter of opinion. I find having Bitcoins to spend quite useful. I found burning out two cpu's while folding@home not quite so useful. Wanting to contribute to the greater good is all nice and dandy, but i see no need for it here. You want to help the world? find away to cut the population in half without killing anyone. that would be a help.
It care if the result of the copmputation is useful because this is the difference from bitcoins and gold I can use the gold also for its intrinsic value ( building electronic circuits etc...) I can not use the result of the computation of bitcoins except as coins