Minecraft wiki entry for Hunger_management, associated text: Advantages:
Easily obtained, either when fighting zombies, or after they burn in the sun.
Rotten flesh can be a good emergency food when no better food is available: If many pieces are eaten at once, the hunger effect do not stack up, instead it will last only 30 seconds from the last piece eaten, and consume less hunger than granted by a single piece of rotten flesh. Accordingly, eating multiple pieces of rotten flesh will still leave the player less hungry and perhaps allow some healing.
In combat, eating rotten flesh is a good way of keeping your hunger topped off so that your health keeps regenerating, without wasting better quality food.
Rotten flesh can be used to feed and breed wolves without poisoning them.