A Tampa Two (Dallas Deuce?) defense is much more conducive to success in today's NFL. When you play man to man defense, you don't put your defensive backs in position to create turnovers, and that's the name of the game on defense now. Defenses are not allowed to simply shut down offenses these days due to the rule changes and proliferation of spread concepts, so you have to force turnovers to be effective. Ryan's defense was among the worst in the league at doing that.
Conceptually, the move makes sense, as you bring in the architect of a defense geared to doing just that. The concern is the age factor and how much drive Kiffin has at this point in his career.