Aside from std.zh, I haven't seen a single .zh file used, but I'm thinking I might start.
When I write scripts, I always try to include lots of options in the form of constants. It would make sense to put those into .zh files. Possibly the setup instructions, too. That would separate the parts that quest makers should and should not be directly concerned with into separate files, and it would allow settings to be changed without having to reimport the script. The main downside is that editing from within ZQuest would require extra work; you'd want to edit the .zh in an external editor. Also, some might be confused at first about which file to import, but I don't think that's too big a deal.
Does that sound like a good idea? Anything else they should be used for?