So, in practical terms, how would this be more secure than what we have now? The main crux of PRISM is that they have highly automated mechanisms of accessing user data from many major internet companies. If you store your private keys alongside that, what are you trying to protect against?
The point isn't to hide data from the NSA. The point is that widely-used PGP would be really useful for all kinds of reasons, but that we don't have it because it would be inconvenient for the NSA if we did (they wouldn't be able to read the world's email, e.g.).
In practical terms, it would mean we could talk with physicians, brokers, banks. We could sign documents. We could get rid of nearly all spam. I mean, the advantages of widely deployed PKI are MASSIVE. And the quickest way to get there is to have webmail providers deploy it.