We have customized the addon to allow 'Registered Users Checking Out As Guests'. Please contact us here for more details.
I bought this addon and it really has improved the checkout, customers seem to be taking to it as well.
It's great to have the checkout work regardless of registered / previously registered / guest - they can just checkout how they wish without being given error notices to login if they don't want to or have forgotten they have registered in the past. The cart doesn't second guess their choice, it just lets them order which means we get paid and the customer isn't left confused why the cart won't take their money because of their email address.
Thanks cscartrocks!