Yes but...
Isn't it more a matter of 'why can't you have a username and password account without storing emails relating to it?'.
In that configuration you could have mails forwarded from that account to another for reading AND be able to login with the requisite username and password to...
I still don't understand this...
So why can't I just set up webmail.domain.com and *.domain.com?
If I set up a second subdomain z.domain.com for my wildcard domains won't I have to upload the same website (set of files) to www.domain.com and z.domain.com???
If so, that is extremely lame...