Right, but as I said this sort of steals a mailbox from the site.
The limit you set on a site is in number of mailboxes which I, and probably most people, interpret as the number of real life users you get with your site.
Using a mailbox to facilitate an alias that is forwarded to more than one person will mean that if cusomer X has a site with 5 mailboxes and 5 users and need to set up an alias for email@example.com
they will need to raise the limit on the number of mailboxes that is allowed. This is of course not something the customer can do themselves which means more support for me.
Aliases that are commonly used are sales@ info@ support@ webmaster@ abuse@ postmaster@. It is not uncommon that these types of addresses send mail to more than one account. If more than one person is to receive mail for each of those addresses we need 6 accounts with forwards set just for these basic addresses.
In ISPC today there is only the aliases as alternate names for an account. What is really needed is to be able to set up an alias that you can add recipients (accounts) to.