If someone is stupid enough to download mIRC from someplace else then the official website, I doubt it would make a lot of difference whether that mirc downloads a trojan while displaying "downloading update", or that it just downloads and installs a trojan whitout displaying anything.
I don't think an auto-update feature is necessary though, that's what the email list is for.
Also, it wouldn't be too hard to script, and I haven't seen a addon fot it yet, so I assume there aren't too many people eagerly waiting for this to be added. (I could be wrong about that ofcourse)
