Hi Matt,
An example of what I do.
Once per month I get 75 downloads from emusic. I then select them all and tag them as source: emusic. Tagging starts. And perhaps it does run in the background, but I can see in the status bar that it is updating 75 files on the network and sometimes it is pretty slow (no doubt due to issues we might have with our wireless connection). But I still have to tag those files for genre, subgenre, keywords, country, etc. I'm always afraid of selecting 20 of those same files and tagging them again for genre when I can see that the tagging for all the files isn't done yet. So I wait (maybe I don't have to?). Then I select 20 of them and update the genre. I'm not done yet, but it starts tagging them. So I wait. Then I do subgenre, wait. Keywords, wait, and so on. I would just like to make all my changes to those files, and then just update them once instead of updating with every single change I make.
The most important thing is that my last step is to move/rename the files, and I'm never sure if I can do that while all that tagging is going on, so I wait again for it to be finished then move the files. But... if I were able to make all my changes, move/rename the files, then set the tag update to go, I'd be all set.
Ideally, I would love a manual function that basically updates all tags for files modified today. Then at night when I'm done using MC I just hit that button and voila. Or even a pop-up when I'm about to exit MC that says: you have modified some files. Tags will be updated now and then MC will shut down. Oh, that one would be sweet!