Of course, the number of possibilities are endless. It's possible that we'll have a 10 game lead over the Yankees mid-September so Hanley wouldn't have to play. I'd bet against it, but it's possible.
It's also possible that the Yankees (or any other team) will have a 10 game lead over the Sox in mid-Septemeber so we can sit Hanley to keep his option from vesting. But again, I'd bet against it.
It's possible that Hanley gets 650 PA's and is burned out by the time the playoffs start but even if that happens I like our chances of winning the WS more if we make the playoffs than if Hanley doesn't vest and his not vesting keeps us out of the playoffs. (!!)
Obviously the chances of Hanley's getting hurt increase with every games he plays. The same can be said for every player. I just don't see that as being a legitimate argument for not playing him if he's playing well.
Barring what I see as remote possibilities, if Hanley's playing well and we have a legitimate shot at the playoffs I want him on the field, plain and simple. And I don't care how many PA's he gets.
Just as you don't save a pitcher for tomorrow because it might rain tomorrow, you also don't sacrifice a chance at the playoffs this year in anticipation of next year. You don't know what next year will bring.