I think there can be some mechanism in the game that you have to be in the area of the workbenches them selfs to work as they should. But in general several workbenches works great from mobs not to spawn inside a spiked wall.
They do not block spawners though. It's like more like I have to build wooden floorpanels into a greydwarf nest to strengthen it from not spawning. But it still ain't completely blocking a spawner.