I somehow remember this issue with Dos2, but I could be smoking crack. :P
So let me lay this out for you. Fighter in the door, cleric a few feet behind the fighter backing them up. Rogue off to the side going for sneak attack. Wizard is in the back looking for an opening to sling spells.
First round, enemy archers and mage rush the door, and within 1 round take the wizard down before he even got a chance to do anything....
Now, they ran into combat with a fighter and cleric right in front of them, but the WIZARD, all the way in the back, was their target? They essentially ran to their deaths by getting that close to the fighter and cleric, just to take the wizard out... I think in a real combat situation, and even in pen and paper d&d, you need to deal with the immediate threats first, which in this case would be the fighter and cleric, and then deal with the wizard once the other two are tied up.
I believe this type of AI removes the tactical aspect out of the game by making all wizards top priority regardless of threat assessment. What this means I have to do is hide the wizard for the first 2 rounds, until all ranged attackers are dealt with, then bring him out to, essentially, just mop up.
Not whining here. I just think the AI does need a bit f a tweek.