it would likely be better to save the money spent on the electric power and buy more computing power later, unless the computation is much much more useful now.

In principle, this is true; in practice, saying things like these seems more likely to make the people in question to simply cease donating electricity, instead of ceasing to donate electricity and donating the saved money to something more useful. Installing a program and running it all the time doesn't really feel like you're spending money, but explicitly donating money requires you to cross the mental barrier between free and paid in a way that running the program doesn't.

For those reasons, I'd be very hesitant about arguing against running programs like Folding@Home; it seems likely to cause more harm than good.