I would hate to see priests being ordered to follow armies and act as bonus-givers. That's not what the class is for. I wouldn't mind some sort of bonus if troops follow the religion of the troops leaders or some such, but everything which leads in the direction of "a perfect army would be composed of such ratio of priests, such ratios of warriors, such ratio of infils, etc" is bad for the game.
I don't think the ability to give morale bonus means priests are automatically soldiers, does it? They still cannot be part of an army unless they already were in it before turning priests. It is also quite risky to bring priests into the battlefield, since they are so easy to capture. This would work more like a bonus at home which can trigger interesting RPs between the troop commanders and the priests.
