Then Job answered and said:
“No doubt you are the people,
and wisdom will die with you."
Sarcasm isn't always a good thing, but in this case with Job, it is a sign of his health. He is responding to yet another string of personal attacks disguised as "friendly advice". Sarcasm shows that Job has not been taken in by the cut-and-dried inadequate theology of the masses that his friends subscribe to vigorously. It shows he knows God better than they do.
This delicious dig at their counsel: "You ARE the people... wisdom dies with you..." shows that Job stills trusts God more than the people around him. He may be tired of their approach, but he can still make his case for the truth that God has revealed to him in his suffering.
So the right sprinkling of sarcastic humor punctuates and seasons Job at this point. It is an insight into his soul and though he is suffering, he refuses to change what he knows about God. It is a way to stand for the truth, maintain his perspective, and relieve some of the stress of the constant attacks coming from well-meaning but misguided counselors.