Not OP, but it's 5% + 30c a subscription.
I hear what you're saying and certainly in this space there are great tools and open source packages like Koudoku [1] which make it easier.
Worth noting though that there are multiple services that just handle the user management, etc. and also multiple services that handle the billing aspects. To my knowledge this is pretty unique in tackling both.
It's not necessarily hard to implement these things yourself, but I think it is hard to implement them really well. They aren't the "core" useful part of your service either, so every second you spend messing with them is time taken away from actually making your SAAS more useful.
At the very least, it's intriguing.