You sure?
From all the matches i've seen of him in Sampdoria i never saw him covering midfield and i surely never saw him defend. When the other team's attacking, Cassano is the only Samp player not involved in the defense and he's always in opponent's half or at the center when the ball is in Sampdoria's half. If you check on Samp's counters you'll see that Cassano is alone up front, he recieves the ball, holds it, and waits for Pazzini to make a run from behind
Maybe you got confused with how he goes deep to take the ball sometimes. But Del Piero was doing the same last year when the midfield couldn't do anything, so Ale had to do everything alone. That's the case with Cassano from time to time, when the midfield is clueless.