I alreday hate the muiltiple-choice-feature in the genre-field (in spite of knowing that it makes some sense there) but this feature in other fields would be a nogo for me.
One of the main-features of the tag-panel is the possibility to have a look at the content of that field in a collection of marked files and if you want to set the content equal. This feature is killed by showing a selection of predefined content as it is done already with the genre-field.
My music collection has ten thouthends of artists and probably the same of different composers. What benefit shall a list to select from bring?
For actions to achieve your goal see here: