I'm not sure if I agree with this. I've found that often times when I call a defensive play late in a game that I haven't called yet I frequently get a big play (sacks, picks), even if the play isn't as effective in more frequent usage. And even your best defensive plays can begin to fail if you call them to frequently. I also find that, besides usage of individual plays, varying what types of plays you call in various situations can be quite beneficial. In my current franchise I don't get a ton of sacks, but I frequently get sacks on the first few possessions just by varying my 3rd down playcalling between conservative man coverage, heavy blitzes, conservative zones and zone blitzes. I would probably be even more successful if I paid closer attention to my playcalling tendency in other situations, but mixing up third and long strategies is one area I've found great success with.
You are right about the play knowledge being a big factor, but defensive playbooks aren't that big - it's pretty easy to have your entire defensive playbook mastered if you devote attention to it.