I didn't like Mantis as a character, but I don't think Englehart found his way with the Avengers until he introduced her. If it took bringing in his own character for him to tell some good Avengers stories, then it was worth it. I think we readers sometimes confuse not liking a character with not liking what they bring to the story (Justice is another Avenger I thought was kind of annoying just as a character, but added a lot to the storylines).