Yeah, but that is because of deliberate choices on their part.
An alternative product model for Dropbox would be to partition the product on one or more features. Something like the ability to share files is an obvious choice, as is things like selective sync.
In retrospect it seems obvious to give all the features to everyone, give away a base level of storage and sell storage upgrades.
Before anyone says who'd be stupid enough to sell backup software any other way look at things like Norton Backup[1], where the license is limited on both the number of computers you can install it on and the size of online storage available.
For example, I'd imagine the Dropbox upgrade rate is much higher than for other apps because people will naturally grow into needing it.