There lies the difficulty of actually pulling off the tactic.. Assuming that your opponent will not deny you such an easy opportunity and that the terrain is in such a condition that you may actually execute the maneuver. Anyway, this seems off topic.
I think it is on topic - Swarms are the only real way Saurus are going to get access to Poison. I do agree that it's not really worth it, though - we saw this same situation with VC Bat Swarms, and they arguably provide a better benefit than the Jungle Swarms (ASL) and are more maneuverable (flight). I hate to say it, but I saw that one coming. If a Swarm isn't at least S3/T3 (a la Nurglings), it likely won't see play.