The MiKandi App Store has been doing this for years. It makes microtransactions a viable source of revenue for developers.
The core problem seems to be that credit card fees aren't structured in a way to support small payments (<$5).
But virtual currency has other advantages. For example: points can be acquired via offer walls (no credit card at all); it can be bought as prepaid bundles anonymously (via third party stations); they avoid charge back issues (refund points instead of dollars to the credit card); they can be given out easily (had a crap experience? have some extra points); and can be traded for other virtual currencies (just like real currencies and third party exchanges).