to compare two or more of the same files' tags, you need to use an 'isequal' expression...
[=isequal([rating],[ben's rating])]=0
will return all files with non-matching ratings, while...
[=isequal([rating],[ben's rating])]=1
would return all those that do match.
In your example, [Rating]=[Ben's Rating], MC does not translate "[ben's rating]" but instead, actually looks for the literal, "[Ben's Rating]" in the [rating] field.
-marko.