While Send Horror/Lesser Horror is an anonymous spell with regards to how it is reported, they are not that anonymous with regards working out who actually sent them in all likelihood. Especially if it is still early/mid game, as it's unlikely nations will have divesified their magic options by then to be able to cast difficult duel pathed spells with non-national mages. Even more so with spells from the blood school, as only blood nations will be researching that path early on.
So as EA Mictlan, the chances of sending anonymous Horrors is pretty slim, as you'll always be the first nation suspected and therefore the one most likely blamed. Whereas pulling off Horror sending with someone like Agartha is a real anonymous shot.
And of course, using a particular nations trademark type of attack (Send Horror for Mictlan, Seeking Arrow for Caelum etc.) is a great way of starting wars between other nations
