No siege during pillage?

Just wondering, Why can't you bring siege units along during a pillage so they can attack walls/towers while your units take on the enemy force/raid their resources?

Because its seems rather stupid that when my guys finish defeating the enemy force that they just stand there whacking resource buildings for loot while the enemy towers just shoot away at them.

You think that while my troops were busy pillaging resources my siege could be attacking their towers or something :/

Use onslaught

Yeah I know, In fact I'm changing my question somewhat, Can you use siege to only break enemy stash's after you wipe out the wall/towers? or would they just target random buildings during onslaught and it would come down to just luck :/