Incentivizing users to add their first file and install it on their phone (where it will likely want to slurp up photos, videos, etc. to back them up there) isn't a bad thing IMHO. It drives immediate engagement with the core product experience, and I'd be shocked if they didn't have data saying that people who upload at least one file and have the app installed on their phone convert to paid at some rate above people who don't. Thus, by incentivizing that behavior, they can likely boost conversion rates AND make users happier by getting more familiar with the service. Sounds like a win-win scenario to me. I wonder how many users sign up but don't ever use Dropbox.

You could call it growth hacking I guess. I think it's an interesting alternative to idk, a weekly "did you know that?" e-mail (e.g. Evernote) or a tooltips-driven tutorial, or Clippy.

[1] https://www.dropbox.com/help/space/25gb-promotion