The App Store application also tries to update all the copies it can see. It's not always successful. (For example, it keeps complaining that Xcode on my "Alternate" volume is not up to date, even though it actually is, probably because I updated it using the "Install Xcode" application that it downloaded to "Normal". Xcode can only be installed on the boot volume. Irritatingly, the App Store doesn't tell you which copy of an app it thinks is not up to date.) It did update the copy of "Install Mac OS X Lion.app" that I had squirreled away on a different volume where I thought it would be safe.
But I've been doing this for a long time, not because I mistrusted Apple's installers (until now), but because third-party installers are a very mixed bag from vendors with very different levels of competence. I've even seen installers try to update copies of their application on the Time Machine backup. (Fortunately, TM is very good at protecting its backups.)
Another problem, less common than it used to be, is that an alias my resolve to a target on a volume different from the one you expect, especially after you've restored from a backup. (For example, a restored alias may resolve to a file on the backup.) If you even run an application, it may think its documents are on the backup volume, and you'll silently update the wrong document. Then all your changes get wiped out on the next backup. As I say, that's less common now that Apple has changed the algorithm for resolving aliases so that it gives priority to the embedded symlink over the embedded volref and inode numbers.)
And, there was briefly a problem with Apple's installer, in that it would damage a drive using a very specific Firewire interface chip if it was powered on (it didn't need to be mounted) during an install. Apple fixed the problem, and it would never have affected me since I didn't have any drives using that chip, but I still power off my backup drives before running the installer.
Besides, you're going to power off your backups eventually, right? They're too valuable to leave mounted as a tempting target for bugs and/or operator error. Why tempt fate?
Bugs, like clothing styles, come and go, and sometimes come back again. Just because a problem has been fixed doesn't mean it'll stay fixed. Better to let someone else report the resurgence of an old problem than to experience it firsthand.
