I think Cavindish's big ambition is to get one more stage win this season in order to equal (better?) the record for a sprinter in his debut season.
Winning the TOB overall is probably seen as being highly unlikely, so they didn't kill him trying to defend yesterday. That should leave something in his legs to try and get (what for him) is the more important stage win number eleven.
Disappointing, as this has coincided with his home tour, but if he gets the next win, he will be the record holder and that will have more caché that 'nearly winning the TOB'.