Multi-edit on tags does a replace. Obviously that ought to be the default, but it is very desirable to have the option to add a tag. Some syntax in the multi-edit to allow this would be nice (e.g. "+tag" in the tag multi-edit field).
Sorry, just found this was asked a few weeks ago. That post didn't show up on my search and I can't delete this one.
Suggestion: "+tag" means add this tag, "-tag" means remove this tag, otherwise it's a replace. Multiple tags where some have + or - and some don't could be treated as a syntax error.