No monsters seem to be spawned from spawner blocks.

Steps, in creative mode (game mode: hard, mob spawning: enabled):

1. Create a spawner block
2. Hold a skeleton spawn egg and use it on the spawner block

Expected: Eventually, skeleton spawns

Actual: Skeleton avatar inside spawner block spins forever and does not spawn any skeletons

This also seems to be the case at least with zombie villagers and spiders. I didn't test with others.