The genre field is, I think, the only field with preset values as defined by the mp3 standard.
All other fields do not have any defined standard values.
So you have to supply a list of "presets" yourself. One way to apply a self-defined value is to define a sort of "master record" in the list.
The "master record" then holds the template for each tag field like e.g. artist, album etc.
Then select all files that should get the template value plus the master record containing the user defined value. Then select the template value from the drop down list.
To my opinion any other implementation would call for the realization of the "dwim" concept ("do what I mean"). How should mp3tag guess which value it should write to which file? So you have to do some sort of selection, filtering and sorting to get the right sort of lists.
That's works very well, especially the sub-menu feature.
Still it would be nice if the values showed up in the drop-down box on the tag panel because if the only items that show up there are and it's not that often you need to use those. It would give those drop down boxes additional functionality.
Maybe it's just a matter of personal preference, I'm not how many others would find it useful.