Didn't Federer said he wants to get to 20 slams?. Whenever Rafa is injured his chances if winning a slam increase... a lot, and he knows it. And Rafa, unfortunately, does with relative regularity either misses a slam or play it physically diminished. I really think he's taking this into consideration.
It's taken Federer since the AO 2010 to win his 17th Slam, and he was very lucky to get as far as the final at Wimbledon. Remember the Benneteau match? And there's no doubt in a lot of people's minds that closing the roof was very much to his advantage. Also, leaving Andy and Nadal out of the equation, Djokovic isn't going to go away any time soon.