Yes, I know he is almost useless against SK and that he is very susceptible to resets and tall punish; and even with a defender he is still highly vulnerable to Yrden and CoC. However, if your defender sticks and your opponent doesn't have Yrden or CoC, Cahir basically represents a insta win against the following decks: dwarves, Viy, thrive, Kolgrim, Aglaïs, harmony, druids and about 90% of NR; he is also quite useful against some of the other ST decks and a few more boost heavy SY and NG decks. Most of the decks I mention don't usually run Yrden, least of all CoC (hence why I didn't talk about arachas swarm, despiste being very boost heavy). In any case, Cahir is usually criticised for being too situational; but between being an aristocrat and the amount of boost heavy decks in the current meta, I think it's more than fair to say that this card deserves more love.
Also, the art is gorgeous and he is one of the best characters in the books!