First, you combine CB and S. Second, how many of those were 1st round picks? Artie Burnt and Terrell Edmunds.. THAT'S IT.
You realize that top-tier CB's usually go in the 1st round, right? So you're saying the Steelers should not take an elite CB (Gardner, Elam, McDuffie) because Terry Hawthorne, Antoine Brooks, Brian Allen, and Marcus Allen sucked? P.S. Tre Norwood, a 7th round pick, looked pretty good last season as a rookie and Sutton is a decent starting CB, taken in the 3rd round.
Maybe you should look at every other NFL team's history in drafting players 3rd round and later at any position. You will be shocked to learn that the vast majority don't turn out to be really good NFL players. A hell of a lot never play a down in the NFL in fact.