As long as Egalitus doesn't get feeblemind, or go mute he should be alright. Even if he goes blind he will be usefull in combat. The main spell I want him to cast 'astral fires' is a 100 precision one.
Quite a few suprises happened in that battle. I was fully expecting to get the message that he died of a disease. But since I was storming a castle that round, and i wasn't sure if I would need to fight more then one battle, I had given my mages a few extra gems. And the one mage used it to cast astra healing. It was a good spell to cast since the enemy was also casting posion spells on my army.
Occasionlly the AI does pick a good spell to use. (shock)