Useless in several different ways. I like their assumption that people would buy a 'Door Chain Mirror' as opposed to mounting a small mirror next to the door; I wouldn't doubt that they sell a fair few of them.
You'd still be taking it on faith that a determined intruder wouldn't just boot the door into your face at that point, possibly after lazing your eyeballs.
They may also wear a hat painted to look like what's outside of you house so that you look at your door mirror and think no one's there. Door-related criminals are very crafty.