I'm mixed on this. I agree generally it's bad AI for an enemy to ignore a threat that can still kill them vs. the one laying on the ground. Some enemies it makes a little sense for them to stay on a target until it's dead though (gnolls for instance). It does get a little frustrating to see mass focusing on a single target and then you get caught up in the one character is constantly down and a second one is helping.
Gnolls that are up close fighters I think is totally justified. They are in bloodlust from the abyssal lord they worship and it fits their concept. It makes them a different foe than the archers peppering the the party. They wouldn't have that same perception or blood in their jaws that would drive them to that. It gives the party more complex dimensions to work with - a big one being DONT LET MELEE GNOLLS NEAR SQUISHIES! Thats fun, and Im totally down with that.
