Hold up....what if the words of the curse, while understandable to your teammates, actually have a different meaning?
A good example would be Canadian political speech; they can grievously insult someone with any unaware listeners being completely unaware that someone was just targeted. For example;
"My learned colleague" means "You cheese-eating throwback!" Would someone hearing those words think "Oh yeah, he just got roasted" if they didn't even know there was another translation?
A similar mode of expression could allow the curser to seemingly be holding a polite conversation on the battlefield, while in reality, he is barraging people with horrific hexes and cruel curses. And if the enemy doesn't figure out a way to translate this 'code,' they'll never know if they're safe, because nothing says the curse can't be sent.
Picture this: the enemy general is told someone has a message for him, he has that person brought in, and when that person reads their message, the general inexplicably dies on the spot. When the enemy investigates, they find what seems to be just a normal letter. Since they can't for the life of them figure out why the general up and died at such an odd time, they figure it was an act of God or a coincidence and let the matter drop.
Eventually, this could lead to people being extremely cautious about letters and messages in general, which would of course hamper the effectiveness of intelligence reports.