It probably wouldn't be worth the effort

Yeah, that's the effort involved in backing it up so that it can be restored directly from the backup. As you say, since you can reinstall from the rescue CD, there's no point.

My point, however, was that worrying about time/space as a reason for not backing up the whole shebang wasn't worth it.

Oh, and IIRC, there is no boot sector on the hard disk -- it loads one kernel out of flash, which mounts the root filesystem and then loads another one.