I really dislike formation as they are and also think they aren't really a fun or suitable thing for a rts like 0a.d.. I also disliked the video about cohesion at suggestions for 0a.d.. Not just because of the problems in the pathfinder. Formations may make sense in the real world, when armies meet on a open field with the intention to clash and end a dispute by one big fight. In 0 a.d. the units rarely fight on a open field. Units join a fight from the barracks when trained, units flee from a fight. The player positions bolt-shooters where one wants to control the map, if your enemy sees such a prepared position you better flee and make them unpack and try else where. The hunting and fleeing creates chaos, the fun in this game is to micro and position one self to your advantage in this chaos. Often there comes also scenarios where one player has some soldiers that needs to flee (where they have the lower hand) while others need to attack (where you have the upper hand) just a few houses away.
Where I could see formations useful is as a short one time action as some players already use them. Just before one big charge. But then later on, first the melee gets damaged, then the healers would need to go to them. When their numbers dwindle, they need to back up behind the ranged. Or focus on a hero.
I'm just saying I'm highly skeptical, but you do you.