Yes track artist is difficult as it can be different things, sure for non-classical it might not be the performer but is generally who you would expect, i.e usually the performer unless the producer/writer is more important. But with Classical we dont go for "who is given credit for this" we always pick the composer, and then we always pick the performer for recording artist so its distinctly different.
Relationships are good because they clearly define the role, but the user interface does not encourage their entry, so you cannot rely on them being available.
But a trackartist does (almost) always make some sort of sense in that it should reflect "who is given credit for this" and this in itself is useful, i.e even if you have the various ARs added the trackartist captures "who is given main credit for this" for this track which the ARs do not. To put it another way you may have composer, conductor and performer ARs but to find out who was the main credit on the track you would need the release itself if there was no track artist.
And personally I think the Classical guidelines are wrong they should indicate "who is credited for this" rather than always specifying the composer regardless of what is on the release notes. But seeing that they do deviate dramatically
some sort of flag would be useful.