Not all tackles are created equal. Aaron Schatz of Football Outsiders breaks down whose tackle numbers most benefitted from tackles on completed passes in 2012 in this piece. Jerrell Freeman, Antoine Bethea, and Cassius Vaughn all make an appearance.
On a passing play, there is certainly a difference between getting a sack and taking down a receiver after he catches the ball. If he catches the ball, there's a difference between getting a tackle on a guy you were supposed to cover and cleaning up after somebody else's mistake. And even if you tackle a guy you were covering, there's a difference between a tackle that shows bad coverage (say, after a 15-yard gain on first-and-10) and a tackle that shows good zone coverage (say, after an eight-yard gain on third-and-10).