I'll try to say it in other words:
In ID3V1, there are actually pre-defined genres. If other users use only these, there will be no hickup. If you do not use these pre-defined genres (and V1 tags), but enter another one, the next one to open the file, will see the genre as "Other"
If you use ID3V2.3, you can enter more or less any text and the next one to open the file will see this text also. I've got a source that likes to use "Hip hop" instead of "Hip-hop" - so my library program shows me 2 entries for more or less the same genre.
What I try to put forward: even though you can select from a list in MP3tag, you are still not saved from a multitude of entries for the same kind of genres as ID3V2.3 allows these liberties.