One of the recent PvZs of ShowTime had a nydus worm in it and ShowTime let it finnish to trap the swarmhosts because he damaged the nydus to 10%-ish hp and when the swarm hosts went out, he destroyed it.
So for terran it could be a medivac with 4 marauders in it to damage the nydus to like 10% and when the swarm hosts are out, the zerg has to decide wether to waste some or all of the locusts to fend of the marauders (which can be lifted / saved) or go on with the attack.
It will take some time to get another nydus up so the zerg has to think about launching a few locusts one after the other to secure the retreat nydus to save the (costly!) swarm hosts.
So even if the locusts attack and destroy stuff, the zerg might loose all of the swarm hosts and thus map presence.