I use as much tags as I can fullfill (and have patience to...), option #4 (nope, I'm not the one you asked for, Chris
) although it's more a #3.5.
I don't mind them being changed but making the old ones not spec-compliant seems to me a lot like
anti-matroska; things should be easy to add/replace without breaking old specs. IMHO old tagging should be
deprecated but not
incompliant.
As for which ones I use... I wrote some crappy code to generate the XML file so I added them as needed out of the top of my head: multilegal, multicomments, multititle and encoder related.
Again I don't mind them being obsolete and not handled by any player, but don't like the idea of some older matroska feature becoming spec incompliant.