Does it make ontological sense then to associate a release with someone who had nothing to do with the release itself? Come on.
On the second point I agree: I want to add the info only once, namely in a structured way as relationships. The need to replicate that info in a poorer way into those artist fields which are there for historic sentiment or whatever doesn’t make any sense to me. That is wasted editor time and an additional source for errors and inconsistency. Again: If those artist strings are really needed, then automatically and dynamically create them as read-only strings from the relationships. If needed, enhance the relationship framework (like adding an “important performer” checkbox).
Also, consider searching for all recordings with a certain performer. Currently, I always do that twice: Once using the artist field, once using the performer relationship. In the end I get two overlapping lists which I mentally have to merge somehow. This is because of that stupid data duplication which naturally is done in various inconsistent ways by different (inexperienced) editors.
Everything would be so much easier if there was only a single place for adding and searching for that kind of info.
Moreover, when the relationships are the only place where you can add the performer info, people are forced to use relationships! Combine that with a more relationship-friendly UI and your (very valid) point
should improve significantly.