Cap wise its a great move. Good DEs in FA are expensive. If Holiday and/or Carter play well, they will demand extensions on their two year contracts. If they play poorly, then we need a DE to play on the other side of Taylor. Its a win win. If all goes well, in two years we still wont want to re-sign these older DEs. We need youth in the front seven.
Also, even when we were considered a top defense, imo we were overrated. Getting abused at times when good offenses *needed* to move the ball on us because our offense was doing well in that particular game. Watch old games and you will see our defense play well when the other team was being conservative on offense because the game was a close scoring battle. When our offense scored a bunch our defense would crumble many times, giving up lots of points against good offenses.