That is very true, Zephyr. When you identify some difficulties in your secondary, you can't pull everyone out of the pressure required to disturb a good QB, only to see if you can scatter more people downfield to defend. I saw someone kind of famous (to us) post the other day that what happens at the line has nothing to do with what happens in the secondary. That is ludicrous. If I sacked the QB every pass attempt, no one would ever know if you had anyone in your secondary. There are gives and takes in any defensive scheme. In the SBC especially, right now, we have some decent QBs. Some are the best player on the team by a long shot. There are gaps all over the field on every team. But the QBs are pretty darn good. If you do not give them a pass rush threat, the shift in power goes wildly in that QB's favor. He doesn't care how many people you place in spots downfield. He sees his people in between those open spaces... and you've given him the time to make the connection. I cannot imagine almost any makeup of secondary, recruited from the south, that cannot provide at least enough coverage, to run a standard, somewhat aggressive defense, and not have better results. You must, at the very least, put a guy (LB, safety, CB) and confuse a QB on whether he may be setting up for a sprint off the corner... or a LB (or two) up against the line in a gap blitz threat. You don't have to actually rush most times, but you do have to posture and threaten... and ever so often send the house.
PS This is not a negative post. This is just my opinion of effective and ineffective defenses when facing decent passing QBs.