@Scott,
"IT DOES NOT SAY THIS APP CHANGED FROM 32-BIT TO 64-BIT, thus requiring 10.7."
No but it does say very specifically that 10.7.4 is required. This is on the download page and App Store as well, under Information Requirements. The 32-bit to 64-bit may be the technical reason, but one doesn't need to know that, just that 10.7.4 is needed.
Unfortunately, Apple did make a huge mistake in not posting any of this in the Software Update tool, meaning if you upgraded there, you'd have no way of knowing that 10.7.4 was required if your Mac already was running 10.7.4. Apple has done this in the past with other software, and it's a very valid complaint and problem. I'd recommend submitting feedback on this specific point.
"In over 20 years of IT experience (my former career, and part-time consulting to this day), I haven't experienced this particular problem'
The problem has occured with most major software applications that use their own file formats. I'm going through the list of apps that I have used through the years and every single one has encountered this issue: Word, Excel, PowerPoint, Photoshop, Illustrator, Premiere, Final Cut, Outlook, Filemaker, iTunes, iPhoto... The list goes on, but I can't find one example of software that's been around long enough to evolve their file format that hasn't encountered this. Again, the problem isn't 32-bit versus 64-bit per se, it's about a new version of the app having a different file format and requiring a different set of specs to run on.
This isn't to bash you, but rather to point out the important lesson here of always reading the release notes, especially when it comes to specs and known issues. Since this information is often missing from the Software Update Tool, I'd recommend using the App Store, or getting the updates from the Apple website where the requirements are clearly listed.
"but I practice a sound backup strategy that suits my workflow."
I don't think you do.
I too keep the original image files from my camera in isolated folders independent of Aperture, so I know I can always go back to them. However, if you care about your Aperture library, you really should be keeping it backed up and if you really care about it, you should keep it archvied.
There are alternatives to Time Machine (see Carbon Copy Cloner), but a sound backup strategy requires also maintaining a back up of the system required to access the data. Any new software could have significant bugs, and unless you can revert back, you may find that the backup of your files (library) is useless.
"I don't back up the huge, multi-GB Aperture libraries regularly (though I do periodically copy them to other disk volumes"
This. You should. Do it regularly, and especially do it before any upgrade to Aperture. On the other hand, if you don't care about your library don't back it up. If you don't care about any changes you've made don't do an incremental backup. You can get an external, portable USB 3.0 drive for under $90 for 1TB on Amazon. Attach it before going to sleep, and you're good...or leave it plugged in all the time. This is assuming you need 1TB, if you need less, it's less expensive, but $90... that's buying you a lot of protection right there... for your whole system, not just Aperture.
Personally, that's not enough for me. I also keep an offsite archive and a mirrored clone. The clone is so that if my hard drive fails, I can pop in the clone and be back and running as if nothing had happened within 10 minutes. The archive is for restoring the system back to a point in time or restoring files from a previous point in time.