Many people elect not to create scourges, only because mutalisks are easier to manage. However scourges have there uses, and are very useful on maps where long pathways exits. For example, guantlet is a map where it is impossible to rush someone. This makes air the choice for most players. This is especially true about the Zerg. Scourges are very useful to cut down mutalisks, but this takes careful micromanagement, or else you can end up suiciding several scourges into 1 mutalisk, which isn't very efficient.
Scourges also make great scouts, since they can patrol for incoming dropships, and keep watch from above. This is especially good since you get two out of one egg and they don't take long to produce. On huge maps as green valleys, using a pair of scourges will help to keep tabs on what your opponent is doing, and where he plans to expand.
One tactic that is very effective, is to build scourges to hunt overlords. This isn't a very effcient idea, especially in the long run, since overlords cost only minerals, while scourges cost 25 minerals and 75 gas. However the purpose for scourging overlords is important. This can prevent a Zerg player from creating more units, since they must waste a larva on replacing the overlord. This is especially useful for winning those muta/muta battles. If you manage to take out two overlods, this will put him back tremendously since larvae don't respawn as quickly now, making the creation of units very slow. This allows you to get a head start on taking command with mutalisks. Though this tactic only works early in the game, it's a good tactic overall.
Scourges also make great dropship/shuttle killers. Having a few just to patrol the sides of an island is a good way to stop any drops. Scourging drops would be easier if you had a queen which can parasite the dropship/shuttle. Losing a full dropship/shuttle is so costly to the other person that they must create an escort of some sort to prevent any scourging.
Scourges aren't made to take out battle cruisers as some might think. It would take far too many, to be effective. They aren't good for handing mass ait units as wraiths and scouts as well. Keep in mind a scourge only has 25 HPs, making them very weak, and should only be used to attack lone targets or support in a battle.
