Torment League: What is the purpose behind the ghosts?

I keep seeing these ghosts in the Mud Flats in the Torment League. Never really got to slay one; it keeps getting away. :(

Do they drop good loot? Decent loot...?

Just thought I'd ask if anyone else knows of them and/or had slain one.

By themselves they don't really drop tons of loot, however... by passing through normal enemies they give them a minor buff (green flames above their heads) which makes them drop more stuff, mostly low currency, sometimes gems or rares. And if they get near a rare or unique enemy they will possess it, granting it a big buff to base stats and a unique property dependent on spirit type. When you kill such a possessed enemy it will drop a lot of loot, but the fight can be challenging if it possesses dangerous mini bosses like Kole or Hailrake, or map bosses.

In low levels it's kinda hard to get them to possess something because monster density is too low, but in merciless difficulty areas and maps it's very easy to do so, you can either chase the spirit around or try luring a rare enemy to it. Of course, once a spirit possesses an enemy you got all the time in the world to kill it.

At the beginning I've had problems to kill them in time too.

Movementspeed on boots as well as quicksilver flasks helped a lot.
But they still sometimes get away especially on big, open maps.

Killing them is often a waste of loot. Run them through as many mobs as possible and try to make it possess a rare enemy afterwards. You'll have a loot more loot pinatas running around that way.

In my experience they are there to make you use all your wisdom scrolls up. It's fun when they posses a map boss though.