Wooden wall towers are hardly more than a token defence. Their field of fire is roughly 60 degrees (i.e 30 to each side), their fire frequency and damage is rather low. Compared to what the stone wall towers can do at least.
They can be a little more effective when the enemy approaches the wall at an 45° angle since some of his units will have to pass by the wooden wall towers. I've seen them burn rams on that occasion, although very rarely.