Regarding the issue of why should one give away part of his computer that paid for, I think the answer is simple, and it's because he is not giving it away to someone else, but to a global community that he's part of it, so still he has interest in that part of his computer, and will benefit directly from the power of super computer, and indirectly from the results of the works of others.
On the other issue, if a computer is down then I think the data should be redundant like resolving domain names in DNS servers.. and the system should be able to automatically re-scale (whether down or up).