The reason I wanted to give Posthaven a go (not having ever really used Posterous) was the admirably simple pitch for the business. To be able to claim to offer a permanent repository for your posts there has to be some way to pay for the servers and the storage. Paying to post makes simple sense.
I was also intrigued by the idea of having to stake a small amount of money to enter the beta rather than comparatively stale idea of simply registering an email address.
I see Posthaven and App.net as real tests of whether people truly want to be customers or in fact they are happy to be the product.