Edit: as of 1.11 spawners are completly broken for rendering

1.11 Renders As Pig Summons Zombies
/setblock ~ ~1 ~ minecraft:mob_spawner 0 replace {SpawnData:{id:"Zombie"}}
Screenshot
http://imgur.com/A1Gln6r

1.10.2+
No Proper Definition for passengers in mob spawners always creates them from nbt rather then my intention if doesn't have any other tags to create them by name and have the on spawn with egg interface giving equipment and specified properties randomized. This is where the old tag Properties:{} would be nice to differentiate the difference between the two. I suggest fixing up the /summon command as well while your at it . So it would work like /summon mob ~ ~ ~ {,Passengers:[{id:Skeleton,,Passengers:[

{id:Skeleton}

] }] Since neither one has the properties tag it will create them by name and use the on spawn with egg interface giving the skeletons their equipment.

Doesn't Render the entire Mount only renders bottom mob in mob spawner

Command
/setblock ~ ~+1 ~ minecraft:mob_spawner 0 replace {SpawnData:

{id:Zombie}

,SpawnPotentials:[{Weight:1,Entity:{id:"Skeleton",SkeletonType:1,Passengers:[

{id:Skeleton}

] }}]}
Screenshot
http://imgur.com/a/6JJ4P

I noticed that 1.8.9+ didn't have proper jockey support definitions only a fixed nbt section of spawners. This means if you have the passenger tag it will be that same mob created from nbt every time when There is no dynamic support for creating the mob spawner if specified no nbt for the rider to create the entity by name spawn in world with the on spawn with egg interface. That being said I updated the world to this version with a previously mounted spawners and It didn't render properly. It seemed to only have the top mob render sitting down rather then the entire mob jockey. Also I am not sure if this is still an issue but, A mob spawners once goes to spawn potentials never goes back, B spawn potentials doesn't support individual spawner tags such has delay for they individual section so I can configure how fast each one is dynamically for maps and mods (I am making a mod and I found many problems with your Tile Entity Mob Spawner and Mob Spawner Logic as stated above.