We don’t show you personalized ads based on your content from Drive, Gmail, or Photos.
That's cool. Seems ok to me.
Then there's this Google Drive Terms of Service which reads pretty ok to me, too. There's this line:
[...] We will not use your content for marketing or promotional campaigns.
Cool. I mean, yeah, they could but they chose not to, so, yeah. Cool.
[...] As described in the Google Terms of Service, your content remains yours. We do not claim ownership in any of your content, including any text, data, information, and files that you upload, share, or store in your Drive account. [...]
Looks pretty good, I don't give up ownership when using their services. Important to clarify.
Then there's the next paragraph:
We may review content to determine whether it is illegal or violates our Program Policies, and we may remove or refuse to display content that we reasonably believe violates our policies or the law. But that does not necessarily mean that we review content, so please don’t assume that we do.
So, in order to distance themselves from any content that's hosted on their servers, Google reserves its rights to, occasionally, look through your stuff an check whether its good or evil. Cool. But please, I mean, pretty please, don't assume that Google ever does that.
The fact that Google chooses to not go through my stuff is, to me, not good enough, though. So, if i choose to use Google Drive, for whatever reason, I prefer to know that Google cannot touch my stuff, ergo I use strong encryption. It's not tinfoil hat thinking, I just prefer control over trust.
Just to be clear - I am not stating that Google actually goes through your files on your Drive account. But the fact that they might choose to do so at some point in the future creates some level of discomfort in me.