To pyramid on Kamamura and Emerson, it should be noted that while horror marks and curses are permanent, neither one is a game breaker.
A Curse can't be cured, but the only effect a curse has is to increase the chance of developing an affliction when wounded. There are numerous items and spells that already reduce affliction chance (thereby countering the curse) and a half-dozen (or slightly more) ways to cure afflictions once they're suffered. It's easily countered, and in most cases outright ignored.
A Horror mark is more dangerous, but though it is a death sentence, it's a slow one. A horror mark has, what, a 2% chance of summoning a horror to attack the bearer each round? Each horror attack should increase the number of horror marks, and thus the chance and strength of the next attack, but it is a process that starts quite slowly.
Add to that the fact that many spells to summon horrors have the built in danger of marking the caster, or a chance the horror will attack friendly troops instead... These are also, despite how deadly they seem, some of the least useful spells in the game. Cast horror marks in any sort of pitched battle is simply a waste of your time when any other two-bit spell will have an immediate and often more powerful affect. Gift of Haven, which will outright kill a dozen troops, or Horror Mark, which might show results in 20-50 turns? I know which I would rather use.
As has been stated above me, horror marks are in place to act as a counter to a much more dangerous and game changing aspect: SCs. It's one of the few counters available when you're too far behind to field your own SC or a large communion. And even then, it's a slow death, a ticking clock. It's a cold comfort, knowing that the enemy's Prince of Death or Earth King will die sometime within a hundred turns, when he's still cutting a swath through your troops and gobbling up territory.
I understand that some people can't stand that sort of asymmetrical gameplay, just as some of us love it, and that's fine. But implying that Dominions 3 was badly designed or badly developed because you don't like an aspect of it is, frankly, laughable.