Why Apple says to erase your drive before reinstalling OS X 10.1 over a Security Update

Thomas Koons received what we hope is the "final answer" on the question as to why Apple recommends erasing your drive if you want to reinstall Mac OS X 10.1 after installing the Security Update (see previous item). He paraphrases what he was told by a "Mac Genius" at an Apple Store:

"The reason they don't suggest you reinstall OS X over the Security Update has to do with them not suggesting you downgrade the system. They don't trust the Installer to downgrade all the components of OS X to the version you install (essentially like a clean install for OS 9 would do). This could mean a now mixed system (some new components mixed with the install you just did). This can cause system instability and crashes. He did tell me it was safe to install using a full version installer if it was the same version you last had installed. This means you need a 5L14 full install CD to reinstall your OS at this point. And of course this criteria changes with each update Apple makes to OS X. As each version of OS X comes out and we install them, we then need a full install CD of that version to do a safe reinstall of the OS for any reason."

Personally, we find this a bit overcautious on Apple's part. Users having been doing these sorts of reinstalls using the first update to Mac OS X 10.0 came out. While we get occasional reports of problems, we have seen nothing to definitively and consistently link a problem to the reinstall. Still, make sure you have essential files backedup before trying any downgrade install.