MOBs will go after ranged heroes, but it seems to depend on what those heroes did (and continue to do).
For example, in the early orc beach fight (prior to entering the town of Cyseal for the first time), the larger orc grunts get very angry at anyone who damages the orc shaman. You can see them react when one of your characters attacks the shaman - they'll make some irritated comment or turn to face the attacker (even though it isn't their turn yet). I've had them chasing my Ranger around the battlefield before (with my Warrior in tow trying to chase after them) because of this.
That seems to happen with the undead as well. Certain MOBs seem protective of other particular MOBs in their group. Generally, they all seem to be concerned with/fond of the caster in *their* party.
Understanding that, you can often make sure the ranged heroes are not being focused on if your melee heroes are going after the right targets.
