If you take the watch off, it automatically locks. I wouldn't mind it also automatically locking my phone and requiring a passcode instead of TouchID.

There is a VERY limited amount of time in which you can take the watch off and switch to another wrist (like milliseconds, you have to practically be a magician to switch wrists (which I do throughout the day)).

Apple has the watch, they could use it to beef up security for those that want it.