In my mind, Rician, there is no thought process that supports this move. None. If you look at the rest angle, while Lester is at a career high IP right now, the person to give more rest is Beckett due to the injury he's dealing with. If you look at the bullpen angle of it, you want the guy who's more likely to get deep in the game in game 6 so you have a full fresh 'pen in game 7. If you look at the matchup angle of it, their game 7 starter is the one most likely to have an off night which would mitigate Beckett's ineffectiveness if they get into a barn burner.
To me, this smells of nothing more than an ego massage. I've supported him in the past, but if Francona makes this move, and if you combine it with his other ponderous moves this series, I'm firmly in the camp that they need a better game manager. Hell, the egos are pretty much gone. No Pedro, no Millar, no Damon, no Curt, no Manny. There's no need for "Vet's" manager who can manage the personalities.