The only field in the tag-panel that has a list to choose from is the genre-field.
The other fields have 3 features:
- delete the tag
- keep the tag as it is
- choose from a list of entries if you have marked more than 1 file
If you have marked more than 1 file you get a list that is populated with the all the different field-contents of the marked files.
For me personally this is an important feature that I would not like to lose.
About actions for multiple choice read here:
feature request: select from predetermined tag set for custom fields