WYM can't keep people from pasting HTML. It can try to clean that HTML (though it's rather tricky to do this, and given its age I suspect it doesn't). Several of the other editors have special code to try to clean up code pasted from Word; they also include different kind of cleaners, some which are applied on save and some which can be applied on paste.
However, I hate opinionated editors. Legacy HTML is important. HTML with differing opinions is important. If you are doing green-field content development I guess WYM could work, but I'm more interested in content development that builds on the content we already have. The words matter more than the form.