Ask HN: Should I use Dropbox for data storage for my SaaS start-up app.

I'm working on a new SaaS web app which includes the ability to store user's files online.

Rather than paying for S3 storage, I'm considering integrating my app with Dropbox so that users files are uploaded to their own Dropbox account, but the are available in my app. My app would simply access the user's Dropbox storage via REST when it needs to.

I'm trying to think if their are any drawbacks to this other than the user having to initially grant access to their Dropbox account form within my app.

Other than that, this would allow me to have a huge number of users, each with decent file storage, at minimal cost.

There is another drawback that all files would be at the mercy of another company. But if I were to use my own storage on Amazon S3, I would still be storing the data on Amazon's servers anyway.

Does anyone see any problems with this idea?

Thanks.