I'm completely aware that the Linux installer may not be 100% by release, and I understand the benefits of being able to always go to iD's website and download the latest version of the installer and go through the old process. My point is that not including the Linux installer on the CD makes it harder for Linux games (the best ones in this case) to become adopted by those who just want to buy the damn game and play it. Linux has come so far that it is almost possible to do such a thing, and this is one of a few issues that is keeping this from happening.
iD's games are what I consider the gold standard of Linux gaming, and because of this many potential Linux game developers will look at their releases to see how a Linux implementation works (or doesn't). Game companies will look at this not only from a developer point-of-view but also from a consumer point-of-view (since those are the people who will be buying the game), and they'll see that even though iD is iD, even they don't include any way for a Linux user to actually get the game running on their PC. You and I will probably say 'big deal' most of the time and just download the installer whenever it gets released a couple of weeks later than the Windows version. Game companies will look at this as a major example in how far Linux gaming has to go to catch up to Windows, and then wonder (even more) if it's even worth getting involved with.
I think it would be nice to have a Linux installer that does the job on the CD, but that can also check for updates before going through the install process. Even if the update check doesn't have the ability to do anything but notify the user it would allow the user to decide whether they want to continue with what they have or see what is required to get the updated version. But hey I'm not picky... just something that gets the game installed at some basic level would be fine with me.
The game had already gone gold? Well I say the installer wasn't even complete, so how can that be the case.