I don't think I quite follow your thinking there. Core provides every infolabel listed at
https://kodi.wiki/view/InfoLabels (obviously depending upon the view and context of the data required). The skin then chooses whether or not to display that info, and how to display it. Depending upon the item to be displayed (film, tv show, album, artist, song, picture etc), core sets all the labels it knows about. It's then up to each individual skin as to how to use those labels.
With what you originally asked about, which was not displaying the song artist if it matches the album artist, is a skin thing. Core will still fill those labels regardless and then the skin chooses how and if to display them. Its job is to supply as much info as it has for any given item, then it's up to the skin what it does with that.
If an option were to be added so that
ListItem.Artist
was not set if it matched
ListItem.AlbumArtist
then core would not be supplying all the info available and IMO that would be a retrograde step. In short, it is not up to core to pick and choose what information is displayed, that is entirely down to the skin in use.
Make a feature request by all means, but this should be directed at whichever skin you wish to see it implemented in.
I hope that makes things clearer.