Compute time on random home PCs is essentially worthless, or at least worth less than the overhead cost of managing it. And for many users the cloud is about storage and bandwidth as much as about compute.
Really? What about the WoW patch downloader, which saves Blizzard a fortune and requires essentially no incremental management effort? (A modified version of BitTorrent.)
Latency is high, and individual input is low, but for simple applications that may be all you actually need.