I really don't see why in this case. They aren't actually arbitraging ad space that Dropbox would be in otherwise. If anything, it's Google that is getting fleeced $50 a pop. But really, Dropbox is getting cheap advertising for the low cost of hard drive space, Google is getting fresh-faced hackers to try their cash cow product, aforementioned hackers get larger folders, and people looking for Dropbox find Dropbox, so really it's win-win-win-win all around.
When I did this, most of my most clicked terms were for things like "free online storage". "Dropbox" didn't really get many clicks. Though, I have no way of knowing which terms ultimately converted better.
