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.