Until HostBill supports "Post2Host", I will stick with WHMCS and its large selection of community addons. No matter how complete a billing system is, there are always missing features that some must consider a deal breaker.
Seeing as almost all of the large Post2Host sites have pretty much died out in the last year or two because people would rather spend $2 a month for shared hosting, there's not much point in such a module IMHO. That being said, you're always free to script your own Post2Host module -- nobody is stopping you. There's no reason to fault the billing system just because it doesn't come with a Post2Host module by default.