Pay-as-you-go Pricing for mobile app- why or why-not?

I am developing an app to address a latent need in the photo space (and no it is not yet another photo sharing site!). I am trying to think through a business model and the one I want to mirror is the Dropbox model 1. Offer x GB storage for free 2. Charge for any x+ GB (and some premium feature-set) 3. Add free GB by inviting users/sign-ups AFAIK DropBox (and similar companies) offer tiered pricing i.e. $99/year for 100GB etc. But are there any start-ups that offer metered pricing (pay-as-you-go or pay-per-GB-Used). Is that a viable pricing model? I haven't seen any start-ups offering it (other than Telcos, Utilities etc.), why isn't this approach more popular? Would that be more attractive to users than the "tiered"? Welcome your thoughts!