I have used swarms successfully* once. Note my success requires an asterisk.
I was playing with a friend and we had an informal agreement that were both trying to play fun orthodox lists neither of were bringing our most competitive list. He was playing Orcs and Goblins.
I don't remember which unit I was stalling but I was able to stall a big unit with two swarms one in front of the other.
Orc unit hit unit hit the swarm. Beat the swarm. Overran into the second swarm. Couldn't overrun again. Also, via positioning the unit was now in a river and could not march to get out.
This cost me two swarms to pull off and required a lot of luck and a friendly match up but I was able to effectively keep a big unit out of the main fight for most of the game. By the time the unit got out of the river and marched to main battlefield, it was too late to help any of the battles that counted.
Think of this as the exception that proves the rule.
Every other time I've used a swarm, even with an Ark of Sotek Bastiladon, even in casual games, my swarms never earn their points value.
Usually they get obliterated and overran or they if it's a multi unit melee they are just wiped out and this knocks down the CR of the other fighting unit.