There are many engineering, legal, financial, and other benefits to using surrogate currencies. Rather than go in to those, I'd like to agree with you: it sucks for users.
I think that Microsoft can get away with the points on Xbox Live simply because they do have a large enough store. I think that a virtual world game could get away with it for the same reason. But a PayPal like system? That's crazy! I hope it is just a temporary solution for micropayments and they are working out the other problems and will eventually support proper currency.
This was tried a couple times in the 90s and didn't last very long. Hopefully Facebook can make it work; micropayments might create some new business models for content producers.
