To address mink's other question - I don't think a one set defense is possible in D3.
First, the stamina ratings of D3 players are too low to effectively run one defense all of the time. By using a second set, it brings a different mix of players on the field which can take some pressure off some of the thinner positions. In addition, having a second formation allows you to vary personnel so as to take some more control over your rotation (and specialize people a bit - important at D3 where players are usually better suited for either run or pass defense, but not both).
Second, I think two formations lets you better deploy your talent against the varied attacks you will face. You can recruit so that the 4-4 is great against the run and the short-range pass attack (pro-set, i-form, etc) - it can be very effective in shutting down teams that try to mix it up by playing a balanced attack from those formations. However, if you roll the shotgun out against the 4-4 you presumably have your best LB on a WR and a second LB on the TE. This creates some very favorable offensive matchups.
So, is it possible? Sure. Can it be successful? I don't think so. A very talented D3 team could run the 3-4 or 4-4 (the only two candidates I think - you can only use one defense if you rely heavily on LBs) against most competition, but would be destroyed in the playoffs against very good teams.