Banshees are fucking assholes. Why would you ever spawn these wastes of resources?
BASIC: Banshees are the second most expensive unit of the whole game (just after Immolators) and thus you don't want them to be killed right after you spawn them. That is why, as the zombie master, you will never spawn banshees in an open area, without any support from other zombies, because the survivors will own them pretty easily. Banshees are the most effective when survivors are focused shooting shamblers, drifters or hulks. Attack survivors from behind when they're busy and you will pretty much make the inversion on banshees worth it.
Then, you have to know that unless survivors are spread all over the map and alone, spawning only 1 or 2 banshees is a waste of resources, because they will last less than a cat in a microwave. They're only effective when spawned in groups of 5 to 10 banshees. Three or four will always die, but the rest will do the job. If the banshees hurt the survivors long enough you can consider they did their job.
ADVANCED: Ambushes are the best way to use banshees, instead of spawning them just in front of the survivors. That is because of the surprise fact, if a survivor walks safely into a room where there are normally no zombies at all, and suddenly 5 banshees jump off the ceiling into him, he will panic and will get very harmed or even die. So, there are two types of ambushes involving banshees: the normal ambush and the ceiling ambush. The normal one is just the one created with the respective ZM button, on the second menu. You place some banshees behind a door, and set the ambush trigger just where the survivors will walk after going through it, and then enjoy the show. The second one requires you to place the banshees under a solid ceiling, and then use the respective command to make them cling on it. You don't have to place any ambush trigger in this one.
MASTER: Pushing survivors off cliffs using banshees pounces is the most expert tactic involving the banshees. It rarely can be used, but if you're a good scouter you will spot numerous occassions to use this one. If a survivor is near a precipice and you have a banshee spawn point near, create two of them and order them to attack exactly so when they pounce onto the survivor they will push him off and cause an insta kill. You can be sure you will be applauded every time you score one of these. This tactic is best used when the survivors are not paying attention or are in a position where they won't easily be able to dodge the attack.
There is also another kind of master trick which is using banshees to explode barrels with their jump attack. This one is more random than anything, and will mostly occur when you're not commanding the banshees, so it's not considered a "tactic" at all.
Banshees are the most annoying zombie type. They run faster than you, jump onto you from like 5 meters, and hit very hard. So what can you do against them? Easy enough, the only way to sort out a banshee is killing him. They're very weak, and they mostly can be lured while running in circles and shooting around them so they don't pounce on you nor hit you. The rifle is the most recommended weapon against these guys, as it insta kills them. The least recommended is a melee weapon, they hit VERY hard in close range.