Yes you are wrong.
Apple's implementation is capable of handling real virtual domains out of the box. The Mail Services Manual explains how to accomplish this.
If you want to optimise your virtual mail setup right from the beginning read my tutorials available here:
http://osx.topicdesk.com/downloads/
Also if you have a great many Unix accounts, WGM is actually not bad for managing them. If you have accounts you need to import from another system this can be accomplished as well. (SASL/Cyradm on the other hand is a pain, so if you must and decide to install the standard packages make sure you compile them with mySQL support and use something like postfix admin and phpadmin to manage things)
The only reason I can see for not using unix accounts, is if you are hosting many many users who are not in your organisation and would like to give them a web admin option like postifx admin (although many tasks can be accomplished through SquirrelMail plug-ins as well).
Alex
