I would prefer it that way, because what if you have a server with 10 domain pointers, most clients register lots of tld's, and the client moves from his own mailserver to google. I (and i think you to) don't want to change the mx records for all this sites.
You add the domain as a "domain pointer" and it behaves as an alias for the main domain, even the mail behaves like this, so why doesn't the dns do this?
In my opinon it should inherit the records from the main domain.