I agree that I wouldn't want to give up Mason unless it's a no-brainer of a trade. With TD, TP, Manu, RJ and even Dice out there, Mason can thankfully move back to the role that best suits him - spot up shooter.
The problem last year was that he was being forced to create offense. He doesn't have the speed to turn the corner off the pick-n-roll and drive inside, so he did the next best thing which was to shoot the 3 off the screen. That's not a hard play to stop, and that's what happened in the playoffs.
This year, all he needs to do is let the others (and you gotta think that there will be at least 2-3 other players on the floor with him who will create) get him some open looks and continue to play some of that improved defense. I think he can excel in that role. A 45% 3-point shooter with decent D is a valuable complementary piece