Spawners now have two additional Calibrations:
Mobs at a time
Mobs at a time mean that this spawner can only have X amount of mobs active from this spawner at a time.
So a zombie spawner with this setting on 5 will spawn 5 zombies, but no more until one of them is killed.
This would be an incredibly useful feature, for both mojang and mapmakers.
Say you wanted a pathway to be constantly blocked by exactly 20 skellies.
You could set the spawner to 20, and it would keep 20 skellies in the corridor, replenishing them at the rate set.
This would also be useful in terrain generation, say in villages.
A spawner could be hidden under each house, and so the population of the village could be kept at a certain number, so 'wipeouts' would be difficult.
In fact, this makes it easier to establish automatic iron golem replenishment system. Right now, either you make them yourself, which sort of ruins the fun, or you make a spawner and 20 end up spawning, unbalancing the fight.
Instead you could set this to 1 and one iron golem would spawn, and another wouldn't spawn until that one is killed.
Mobs total means that once you get near the spawner it will spawn X amount of mobs, and then break.
This would be extremely useful for one-time unavoidable traps, or if you wanted to set up a boss fight or something.
So if you wanted a long pathway filled with 100 zombies and then, when you get to the end, a boss appears, it would be relatively easy to do without redstone.
No, they can't. He's just really REALLY good at judging how long people would be in an area etc. They work in a COMPLETELY different way.
1. X amount of mobs - Just limit entity cap to X. Problem solved.
2. Mob total and break - Check how long it takes to spawn, make a timer and blow the spawner up after the exact time is up. You can adjust the spawner timer to make this faster/longer and easier. Problem solved
Now what can you not do?