There is a legitimate debate to be had as to whether or not Mike Yates actually qualifies as a Companion (with a capital 'C'). No matter which side of the issue you come down on, there remain two indisputable facts. 1) He did appear in more episodes than Susan (and the Sixth and seventh Doctors, for that matter!), and 2) At no point are fans expected to deliver an official report on who's a companion and who isn't to the international Doctor Who committee in Bern so it's really not worth worrying much about. Official companion or not, the real problem with Mike Yates can be boiled down to three words. He's not Benton. After a season(ish) of charming and loyal Benton, we meet his superior officer. He's nice enough, if a little officious. Then Day of the Daleks happens and he steals Benton's dinner. Then he betrays everybody a couple times, leaves the army, sort of gets Jon Pertwee killed and vanishes forever. All that screen time for Mike Yates and Benton never even gets a first name. It's just not fair.