Even against less microed zergs....Scourges can just flood in and blow up the Carrier.

With the Queen's Ensnare, Carrier is nothing more than a minor irritation

In theory, plague would do wonders in destroying terran buildings, but practically, you rarely get the opportunity to do so. At mid game before you get defilers, zerg is pretty weak against a mid game terran army as mass medic and marines or mass Goliaths would chew away you unless you can hold your base before your defilers come out. When defilers come out, players should be about finishing the consume research, which is crucial as dark swarm is drastically needed to counter and defend against terran tech. Later parts of the game for zerg vs terran solely depends on the player expanding properly and dark swarming properly. As almost all of terran units are ranged, dark swarm practically makes terran units useless under the swarm. Not to say that plague is useless, but progamers rarely use it on buildings as defilers are always in the battlefield throwing out dark swarms and plagues on enemy units.