On further exploration, the MTP mode difficulties (that made me start playing with MSC) may in fact be a problem with Media Center 17. I noticed the problem after updating to 17.0.91 (but I hadn't done a sync in a while); I rolled back to 17.0.85 -- which I thought had been working for me -- and the problem still appeared (which is why I blamed something from the latest round of Windows Update hotfixes). As one last check, I rolled back even further to 17.0.82, and so far everything appears to be working properly.
My symptoms are these: I regularly sync a few smart playlists to my Clip that I use while exercising. The system has been working fine for months. With 17.0.91, I found that after syncing, my playlists on the Clip were empty. Re-syncing didn't fix things, so I reformatted the Clip, and the first sync after format worked fine. The next day, when I synced again, the playlists again showed up as blank -- and files that were on the Clip but no longer on the playlists were not being deleted as they should have been (I have "delete from handheld any files not in sync list" selected). I played around and the problem appeared consistently (working on the first sync post-format, problems on the second and beyond), although the playlists were sometimes just partially populated rather than fully blank.
I have multiple Clips around, and tested a second one to check that it wasn't a hardware issue with the Clip I regularly use. The same problems appear on it.
As noted above, if I roll back to 17.0.82 things seem to be working properly. So...it appears possible that something after 17.0.82 is causing my sync problems in MTP mode. I'll keep an eye on this and report back in case the problem appears again even on the older build.
[Mods: feel free to move this thread to the portables forum, which is probably where it belongs]