[chance the horse bumps half of my team] + [chance the enemy gets the horse] > [chance a teammate benefits from it]
So I will kill any goddamn riderless horse I see, the closer it is to the area my team controlls, the faster I will try to, because the possible damage, if the horse goes rampage, would be the biggest possible.
If you want to use a horse then buy yourself one, with the new upkeep gold income it's no problem. If you did and lost it: your fault. I hate cavalrymen who get dismounted and run away from every peasant, totally panicking, only to find another horse as fast as possible. Because without horse they are like shit, huh?
Edit: you also pay upkeep for found horses, just to mention!