I use watchers alot and yes you need a mage(or other leader with magical leadership) to patrol with them.
I personally set them as far forward to the centre as possible. Scripting fire closet, they then launch lightning strikes. Have a mage patol who can remain in the centre and buff your PD up with a few spells. This is a very cheap and effective way of boosting a provinces PD.
One final point even with 50 moral they will rout from battle before they are killed.
