He's not bad on defense. Marcelo is bad. Maxwell had real defensive limitations. Adriano plays taller than all the other Barca fullbacks, due to his athletic ability and strength. He recovers extremely well, he is faster than any of the CBs and than Montoya. No, he is not perfect positionally, and he sometimes he gets caught downfield. Not at Abidal's level for defense (who is?).
With the lack of scoring from all the forwards other than Messi, and Busquets almost never scoring, Barca desparately needs someone from the back line to put the ball in the net. Adriano loosens up the opponents' defenses.