You could have iLvl over 9000, and if all you do is spam cure 2, your team is gonna have a bad time. Underleveled/Undergeared largely just means there's less room for error on the part of the player. What that means is, a good player who makes few to no mistakes, can perform equally well to someone who has superior gear. Knowledgeable players will be able to take care of you better except in the most extreme cases of overgearing.