My assumption here is that they cannot be faked. Some projects pay for computation, why don't people game those systems?
It's not a good idea to have an idea rely upon "if it hasn't been done, it can't be done."
The only system I can find that openly pays for CPU time is cpusage.com
There is no discussion on how they secure against cheaters. In principle, they cannot verify the result of an arbitrary computation without themselves performing the computation. It is likely the client is not open source because it would be too easy to cheat.
Folding@Home keeps the client closed source because it would enable easy cheating. I doubt anyone would trust a closed source coin.