I'm surprised not only that the original simple solution doesn't work, but also that people don't expect it to work in tags.
Actually I am less interested in the real requirement here than the MC capability, which I could apply to other requirements. As always, the simplest solution is always best, and the original expression applied to a tag would have been the simplest solution. I guess I am too used to working with databases, and the MC library isn't really a database, in the sense of Oracle, DBII, Progress, etc. Or maybe the value [varies] is in fact calculated on the fly for presentation on screen, and is never actually written to the tag.
I agree with your general idea in computing that simple is best, consistent is best, and general functionality is better than specific add-on functionality. I think that's what you were saying anyway.
In this case, you're missing something about how MC's tagging works. So it seems like it's broken, when in fact it's working exactly as it's supposed to. Your last sentence above is the key:
Or maybe the value [varies] is in fact calculated on the fly for presentation on screen, and is never actually written to the tag.
As I understand it, [Varies] is just a display that tells you: "This field has several underlying values for the files that are selected." When you write an expression in *another* field and use it to test, it looks at EACH FILE INDIVIDUALLY. The tests never see "[Varies]" because the tests are applied to every file and see the individual values for every file. That's why putting something like "I really love [Album] don't you?" in a field works. Because it sees the individual value for [Album] for each file and can substitute it.
The question then becomes, is there some way MC could be modified to test for [Varies] in an intelligent way that wouldn't break the existing functionality? ...and would it be useful for more than just cases like this one?
Right off, I can't think of anything this would break. But it seems like it would be weird to code it. Though [Varies] is displayed now so... It still seems like it would be some strange special case code to make this work in the Tagging Pane. Would it be useful for other stuff? Almost certainly.
Note that the original requirement of just showing something based on [Varies] does seem to work in an albums view. Also note that to do any kind of sorting, etc, you would still need another field like I've suggested. Unless I'm missing something.
Interesting problem and discussion.
Brian.