I've found very useful to have multiple variants for saving tags in ID3v2. But saving not ISO-8859-1 characters with "Save to ID3v2.3 ISO-8859-1" options enabled gives unreadable result. I suggest slightly change behaviour of this action - add (swithable) warning about losing information after such conversion. It's also possible add new option like "Save to ID3v2.3 ISO-8859-1 when possible" and authomatically decide about the possibility of lossless conversion to iso-8859-1. In latter case current behaviour better rename to "forced" variant (w/ or w/o swithable warning).
There are many english singing bands have only ASCII letters (rarely iso-8859-1) in there names, album and track titles. Unicode is not needed for such tags. But for national bands in non-english countries Unicode in v2.3 or v2.4 tags is required. And it's not very handy switch back and forth the variants of ID3v2.3.
For me UTF-8 is the best solution but some (really many for now) hardware players do not understand ID3v2.4 yet.