OK...to start off, this should not be the case if you are fighting 'smart' foes (e.g. NPCs and maybe robots) Because they will figure it out that you better kill that healer first or you'll never win the fight...
...but when fighting animals, creatures, brainless mutants, this should NOT be the case.
Yes, this would be different types of creatures would have different AI. Wow, is that a neat feature or what?
I could see why 'herding' animals (like birds...err flocking animals) should fight together if one is attacked but snakes are NOT social creatures and I don't think they should all come attack me just because I started attacking one of them.
This would make fighting/hunting much more useful...and maybe give the Adventurers an ability to see if a creature is a social creature or not.
Just imagine, you see a field FULL of snakes and your team says, "No way guys, time to move on" and your adventurerer says "Hold on cityfolks, them there snakes are loners. We can take them one at a time."
Ah...thank goodness we had someone who knew the land.
I know this is a revolutionary idea...different mobs behaving differently (and not by just aggro levels)
Think they will get to this sometime before the storyline finishs?