You can run Cover-2 and Cover-3 all game if you want to, and if you're sound at it, it won't get exposed like we do on a regular basis.
The key is to make your coverages and fronts LOOK different every play. I can run Cover-3 every single play but if I make it LOOK DIFFERENT every time then it's gonna ***** with the QB's pre-snap reads. The first thing a QB is looking at are the Safeties. Their depth and alignment will tell the QB what coverage the defense is in.
How do you think offenses would expose a "vanilla" defense? Well let's see. If a QB sees Cover-3 pre-snap then he knows that the seams are open and the flats are soft. Most likely he's gonna look to throw to one of those spots. In Cover-2 the the soft spots are down the seam between the Safeties and the "hole" down the sideline between the CB's and Safeties.
NOW...
No matter what coverage you run as a DC you know what you're giving the offense. You know where the weaknesses are in that particular coverage. If I call Cover-3 I know that I'm gonna be vulnerable in the seams. So how do I stop the QB from throwing there?
I have 2 options...
1) Don't give the QB a pre-snap read of Cover-3. Show him Cover-2 and then roll to Cover-3 after the snap.
2) Run a Cover-3 that has different principles and different post-snap reads than the standard Cover-3. Example: A version of Cover-3 that I like to run has my flat defenders (the OLB and Strong Safety) cover the #2 WR if he runs VERTICAL (down the seam) or OUT (flats). This gives the QB a Cover-3 pre-snap read but when he attempts to throw the seam route there's a defender covering him. THE BASE PRINCIPLES OF THIS COVER-3 COMPLETELY TAKE AWAY THE COVERAGE'S MAIN WEAKNESS. (the seams) This leaves the QB with only one other option, the flats to the #1. (i.e. a hitch route) The first time I ran this coverage my Strong Safety got a pick-6 cause the QB read Cover-3 and threw "hot" to #2 down the seam.
The same thing applies to Cover-2. You can run a basic Cover-2 as much as you want but you gotta make it look different. Have your CB's fool around with their alignment. Show press-man and then back out just before the ball is snapped. Align in a standard Cover-3 alignment (7x1) but then come up into a Cover-2 after the snap.
Principles > Pre-Snap Alignment > Actual Play Call
Once you get those coverages down perfect then you can start creating different ways to bring pressure.
We're not being exposed on a weekly basis because we only run a couple coverages. We're being exposed because WE DON'T DO ANYTHING RIGHT. We're not good at any one thing. That's why offenses can take whatever they want against us. We're not good at Cover-3, we're not good at Cover-2, our LB's are lost in coverage, etc. It shouldn't take 3 years to master basic Cover-3, Cover-2 and Man. I got high school kids to master it.