The likelihood of someone going to great lengths to break through Google's security is rather higher than someone going to great lengths to break into your personal computer (assuming you are at least capable of securing that against generic threats, which really isn't that hard for technical folks).
It's the agglomeration of lots of people's data that gets dangerous/interesting. And seemingly boring data like your browsing history can actually be pretty revealing -- ah, hey, I notice you regularly sign into the Bank of America site, plus you have an account on the forums at (easily hackable site)... might your username and/or password be the same? Ah, you sign in as the admin on an "anonymous" blog... and you mention your first pet (common security question) and link to your favorite uncle's blog, but he has a different last name from you... so is that your mother's maiden name? You see how this works.
If some clever had access to this level of data on lots and lots of people, they wouldn't strike gold for everyone, but there would be quite a few casualties.
All this to say that there are some very good reasons to avoid letting corporations gather lots of wide-ranging data about you in one place. The intentions of the corporation don't actually matter as much (though obviously if they're selling your data, the risks are higher); it's how valuable that data stockpile is, and to whom.
If you just have your own data, and you manage it yourself, it's not worth it for someone to put much effort into hacking you personally, so you're in a much safer position.