Haha
So, have you tried all of them? Why would you recommend doing it manually? There are several factors that have determined MY use of an automated billing system, and the use of others too, they are:
1) Online Billing - Client like it, why do you think there are so many banks and other businesses that are allowing the option now?
2) Simplicity - Its much easier (talking about ModernBill) to automate the signup process and setup the users account at the click of a button instead of having to login and set the client up to active and then having to login again, to WHM, and set the client up and specify everything
3) Variety Of Billing Options - Clients LOVE a variety of billing options. I allow many different kinds including credit card, PayPal, StormPay, personal check and even Western Union money order for international clients. I have allowed even other kinds just to suit clients including signing up for that StormPay account so my client can use it.
Simply put its easier, more cost effective, and smarter to use an automated billing system, and that is why so many people are doing it. If automated billing systems werent needed, there wouldnt be so many out there or in current development.