This works.  I tried several remedies for Error 12 posted on various message boards.  None of them worked.

It's not easy:  I had to run not only the save files and the library, but also all the custom content I'd downloaded through the years, through the decrapifier.  And as bloated as my save files were before it, I don't know why it was needed as I have 12 MB of RAM (although I think TS3 as a 32 bit program can use only 4 MB, but regardless the task manager showed the memory usage when I went out to Edit Town at about 2.5 MB), but I did have to do it.  It took me over an hour to get everything decrapified plus finding a few pieces of custom content that wouldn't cooperate (Process Monitor showed TS3 looping on the hair and paintings of 2 creators).

But...it was well worth doing, as I was hitting Error 12 anytime I played for over 2 hours at a time, and now I just played 4 hours (saving in between because I was unsure it would work) and never hit it.  The save files are much smaller as well, though in today's age of cheap hard drive space that isn't much of an issue for me (or likely for most people).  But getting it to save certainly is.  I recommend this method to anyone who's getting repeated Error 12's.

As was said in the post early in the thread, every bit of custom content-- not game mods, but skins and items (I download a lot of "paintings" because TS3 comes with so few, even including expansions), has to be decrapified or it won't show up in the game anymore.  It can be dragged to the decrapify file in Windows, even selecting multiple files at a time, so it's not as bad as it sounds.  It still isn't fun to do the decrapification, but if the E12 is ruining it for you it definitely has seemed to fix that and you'll have a lot more fun in the end by using this fix.

Edit:  It didn't work the miracles I thought it did when I wrote the above.  Last night, I Error 12'd again when trying to save.  Then in something that was almost certainly a coincidence but was still a pain, after I finally gave up and quit without saving, I couldn't start up again.  Origin said it couldn't reach the net and that my first login had to be online (it should have had my login data saved and known I'd been on many times before, and I could even log in to the Origin site on my browser so it should have been able to reach it, anyway).  But I did just now manage to reload my game.  I did think that this was a panacea that would end the E12's (for at least a couple months, until the files got crapified again and I just had to do the same again), but now I'd say it only helps a little.  It's still better than nothing, but if you have a lot of content that also has to also be decrapified I'm not as sure it's clearly worth it as when I wrote the above.

To sum it up: since I don't give a damn and actually find it kind of interesting to see your desperate fails to offend me then YES, it was worth the "effort". Had I truly wanted AM I would of had it in my game right now.