When testing spawn conditions regular Spider Spawners include the count of Cave Spiders when testing for nearby entities. This is odd behaviour because the mobs have different EntityIDs and spawners "should" only count entities with the same ID.

How to recreate:
1. Take a Spider Spawner
2. Put the MaxNearbyEntity count of Cave Spiders in it's Spawnrange.
3. No spider will spawn.

http://youtu.be/IcpcITHYOuw

Other hostile mobs do not interfere with the spawning.
Also: Regular spiders do not prevent a Cave Spider Spawner from spawning.