That's depending on the type of siege. I frankly find the open nature of barbarian towns far more suitable for spear warbands.

I simply see neat rows of enemy dead when assaulting a barbarian settlement where they tried to make a stand and were mercilessly cut down by spears.

For civilized settlements, I typically have a barbarian swarm tactic where I have spear warbands with phalanx turned off mixed in with axemen simply swarming through the gates. The spearwarbands then try to engage phalanx when they start tangling with the enemy.

Also, don't forget that spears clip through walls, thus while fighting at the gates, some spear warbands might be able to "flank" the enemy while out side the town.

Once you get clear of the gates, then form a nice phalanx and let the killing machine rip.