The first application is just to burrow all but one of your defilers at expansion as well as a handful of lings, so that way the Terran player has to click individually on tiny little black dots to find them, and spend more actions that are not doing real damage (like irradiating several defilers, managing drops, etc.) for relatively little effort on the Zerg's part, since you can use box and the wireframe click to find the defilers quickly enough. Throw a swarm over them, and they become immune to tank fire as well, and far more difficult to irradiate, even if Terran sees the lings and defiler burrowing.
It can also be used during pushes, counter-pushes and drop harass, which is a bit more apm intensive, but has the potential to be even more so for Terran to clean up and possibly game-ending (if his push is delayed for too long, the critical expo is shut down, plagues over depots and production) if he doesn't devote time to it.
Zerg players: Thoughts? Have you tried these kinds of tactics? Did it feel worth it?
Terran players: Have you ever seen this kind of play? What was it like to deal with, or how much would it really bother you?