I really wish there was a plan between free and 50G (now 100G). Kudos foe the increased storage though. Not having the pricing page live yet is kinda weird.
So do I, and I think 99% of all Dropbox users at some point. Why then, don't they do it? I think, its because this makes getting "free additional space" so very attractive. You envite lots of friends and colleges in order to grow to a few GB [1]. I would guess this is one of the biggest reasons for Dropbox's success. [1] For instance, I have 8.4 GB by now, without cheating. This more or less meets my needs.
User acquisition costs for a product like Dropbox can be fairly significant, especially if you're going toe-to-toe with everybody else in Google.
On the other hand, a gig of storage borders on free for Dropbox, but the marginal utility of that storage is pretty significant for the user, providing a strong motivation to actively recommend (an admittedly excellent) service to their friends.
You get more storage for 'free', Dropbox saves a bucket of money on advertising. It's genius.