I think the primary benefit is that in theory you can cut Google off at any time. If you disable the key service they can no longer decrypt your data. So if you

I think the primary benefit is that in theory you can cut Google off at any time. If you disable the key service they can no longer decrypt your data. So if you decided that Google is no longer trustworthy you can leave and they can't access your data. Of course this is sort of an odd game where you need to cut their access off before they backdoor it, so you have to somehow predict that Google is going to become malicious and beat them to the punch. If you a reacting to something that they are doing it likely isn't helping much. Another possible advantage is that you could potentially have logging on key access which could give some idea of data usage. So if Google starts requesting keys for all of your stored data then you can be suspicious that they are siphoning up your data. (Or doing some background maintenance? Who knows?) In practice this is probably mostly checkbox theater where it is a feature that Google and their users can list.