And what is that number-crunching going to? I consider it a donation to charity.
More like donating to science at best. If you really want to donate to charity, find someone LOCAL who needs the money and help them out. Charity from-a-distance is like government from-a-distance: fraught with waste. Charity is best done at home.
Anyhow, the chance of "donating $20 to science via distributed computing" doing any actual good is pretty slim. You'd be better off donating the $20 to some cause you believe in -- a scientific foundation if you prefer.
There's a lot of overhead in distributed computing projects, and only certain problems can be broken down into segments where reliability and resiliency aren't important. Remember, any piece of the puzzle can go "poof" if the person with that slice decides to quit. So the pieces of the puzzle can't depend on each other too much.
Many number-crunching scenarios could never be written for a distributed computing platform. Too much memory is required, the calculation can't be easily split up, etc.
. A no-hassle, $20 a month donation to a distributed computing science project is IMO a worthy cause. Due to the waste and expense of distributed supercomputing your $240 a year only buys you a fraction of a letter in one of those papers, but nobody can undo the science that has been done.