Blitzing can put pressure on the QB and cause him to make a mistake. That said, if your DB's are average or worse, and you leave them alone to guard the WR's, then the risk isn't a long sustained drive that ends in a score - the risk is a quick strike that results in a score.

Generally speaking - most teams aren't great at multiple sustained drives of multiple plays to score lots of points. So playing it safe, so to speak, is also relying on human nature of making mistakes - an errant or missed pass, a bad route, a missed block, etc., that results in the team not sustaining a drive and having to punt.

Good teams can sustain multiple drives and this approach doesn't necessarily work. Teams with a good run game are good at sustaining drives if you don't take it away - which was the issue with some of BYU's games last year.

It can be frustrating seeming to watch a team have success while our defense just lets them. I lean towards being more aggressive rather than waiting for them to screw up myself. But it's hard to argue with the results last year of winning all but one game - and being in position to win that one. (I do still wish BYU would've stopped the coastal carolina rushing attack - but they were in a position with a chance to win.)