1) don't confuse your customer management with your server management, your customer records and how you interface with them is something that needs to overide possible changes in server software somewhere in the future, your customers will always be your customers, plesk may not always be your server software.
2) Again don't confuse your customer management with your server management, these are 2 distinct areas of your business, plesk have openly said they will not include billing/support etc for end users in their product, so why does billing software have to tie in so clolsely with server software.
3) auto-creation is probably not something you want to go for with the amount of fraud around.
The most important asset of your business is your customer base, make sure you get a product which deals with them today and in the future.
Setting up customers in Plesk is a 2 minute job with the new templates.
I think modernbill is the most robust application. Plesk 5.0 does most of the automation and moderbill is going to be adding a plesk add-in for auto creation of new customers with a click of a button.
Modernbill isnt trying to corner too much, they provide a customer management, system that is moving towards an all in one application for billing, support and account creation and domain name registration.