This is more a question of curiosity...
I installed the WebMail .NET interface today and found that I need to install IIS6 service roles in order for it to install. After the installation I see that the WebMail .NET interface is an ASPX application, so why the need for IIS6 service roles? Isn't everything you would use in II6 available in IIS7? Could there be any dependencies on IIS6 for the ASPX application (I would think not, however...)
So, I decided to uninstall the IIS6 service roles after installing the WebMail .NET interface. Doing this does not break anything, this is what I would have expected since the WebMail .NET interface is a ASPX application. This leads me to believe that there is a dependency in the installation process and not in the WebMail .NET interface itself.
FYI, I did this on both W2K8 R2 and W2K8 x86 OS's. In both cases the WebMail application runs without the need of IIS6 service roles. I explored this because I am a minimalist when runing servers services. If I do not have a need for a service I remove it. In this case I do not need the extra overhead of IIS6 roles if they are not being used.