I think more and more defensive coaches - our staff included - are realizing that frequent substitutions and lots of varied personnel groupings are simply not viable options against the better HUNH teams. LSU is another defense that does this very well. They made one of the more prolific offenses in SEC history (Texas A&M) look pretty silly the last two seasons, and they did it mostly by keeping a lighter, faster personnel group on the field with few substitutions and using a lot of aggressive early-down blitzes. This way, your personnel grouping has a chance keep up with the pace if it gets cranked up, but you also give yourself a good shot to blow things up in the backfield on early downs. You leave the back end fairly exposed and they'll probably burn you with a couple of haymakers, but I think that's the lesser of two evils at this point.
To be fair, this is pretty much the way Chavis defends any offense, which is why he's not without his detractors.