I have more than a dozen user defined fields for my classical music, my artworks and image collections (about 10'000 images). I have user defined fields such as composer, conductor, orchestra, soloists, movement for classical music, period, paintstyle, paintyear, paintsize, gallery for artworks, continent and country for images. A lot of image tagging was done during the 9.1 beta phase and it was really a big improvement. It became so easy and quick to tag images.
Since then I did only little tag work, but from the limited use I did, I got the impression that tagging mode was better organised at the end of the 9.1 development cycle, although they introduced auto-complete in 10.0. But I often found myself to click twice the same tag before MC10 accepted my choice. Sometimes you selected a tag, than the whole thing was reset and I had to tag again. As I did not have time to investigate in more detail, I cannot say whether it was buggy or not. Maybe it has improved since then.
In any case the combination of panes and tagging mode is a great winner for MC.
What they probably need to do is a security flag that does not allow to make tag changes by anyone at party time.