Curiosity has been resolved.
Remember the old song, "You don't know what you've got 'til it's gone"? I'm now back on Plasma as I write this.
I worked pretty hard to give Budgie a good chance, but in the end, it takes more than a better dark theme to win me over. I'm not going to detail all the reasons why I've returned to Plasma, because it wouldn't really matter, and because I don't regret having done this experiment.
I'll just say that the old song was right. 100% right. There were dozens of little things that, taken individually I could have ignored, but taken all together were enough to get me to change back.
As I've mentioned, my Dell laptop makes it trivial to swap disks, so it took me about a minute to go back to Plasma. And most of that time was spent pairing my Bluetooth mouse and keyboard again. (I needed to make Plasma "forget" them before I could get them to pair again.)
But best of all, I now have a fully-configured Budgie SSD that I can swap back into my laptop the next time I feel like trying Budgie again. (Maybe when 10.7 is released?)