In the general case, Dropbox benefits from unused (but paid for) capacity, while users want a fixed price, even if they're overpaying (compared to use). For some products, pay-as-you-use seems better and more attractive, but Dropbox wants simplicity and ubiquity, so I don't think that model would be better for them.

See http://blog.dreamhost.com/2006/05/18/the-truth-about-oversel... for a related discussion.