From using the Defensive Keys at the Community Event, they seem designed for human vs. human games, particularly versus 'prison rules' (cheese, anything goes, etc.) type of players you will likely see online - particularly the Run Left & Run Right that we saw Haumiller use in the FSU video.
We've all played those guys online who run the same plays on certain down & distances. 3rd & short? Speed option right with the WR at QB! Use your Defensive Keys ->Run Right - Boom. johnnywalmart will have to learn some new plays since you just blew up his 3rd & short money play.
The Run Mid & Pass will get some play vs the CPU on 3rd & short and 3rd & long respectively, but IMO the Run Left & Run Right are designed to help shut down human players who run the same dang plays on the same situation repeatedly. As you saw in Haumiller's FSU video (and as he intimated here), defenders go HARD on Run Left & Right. Don't use it if you "think" your foe is running a certain play; run it if you "KNOW" they are running a certain play.
I hope that helps explains it more. Come online play in NCAA Football 10, this will be a widely used and liked feature versus the stereotypical East Room style of play.