[Agema]

I don't think the idea of assassins is that they dash in and murder anything. They can be useful without being successful at killing, simply by being disruptive. Assassins are best at killing unguarded characters, they can even kill a human-size HP mage with not awful odds (mages scripted for battle are often ill-prepared for assassination, especially communioning ones.)

Keeping many commanders and mages alive means bodyguards - not just on armies where their function is particularly vital, but possibly all researchers too. Those troops will be unavailable for frontline duties, and will also cost upkeep probably greater than that the assassin(s). For that matter, one assassin that does successfully damage a vital commander or mage can be the difference between a won and lost battle.