Different auras for different applications imo
If you're a true pure summoner using only demon damage then aoe auras are wasted, because anything it hits is going to be cursed anyway. Well 75% of the time, but its still an aoe that applies it to the rest.
Echoing strikes and blood boil builds need aoe curse from holy damage auras. Maybe the budget blood boil without infinity wants conviction + cursed and rely on the amp procs from attacks alone
Fire warlock clearly wants conviction aura on the cheap
But for a pure summoner who is only attacking with demon then fanat seems valid. I mean, its competing with you using beast mainhand, otherwise what else would you want? Might and holy freeze could be swapped between demon and merc otherwise, and ideally you'd get holy freeze on demon + cursed and might + pride on merc, but hell you could self-wield doom if you really cared about holy freeze too. I don't think the higher level fanat aura on demon is worth much though, because it ONLY gets the half % ed bonus rather than the double-to-self a player gets with it