Sieges are very useful at pinning down the inhabitants of a settlement, preventing them from exiting certain areas and halting reinforcements from entering their location.
So why don't Rimworld sieges do this? Bombardment can be shrugged off due to its inaccuracy and limited explosive range, and in a colony where most things are built out of non-flammable materials (typically when mortars start showing up) it's very rare that permanent damage can be done.
Instead of letting this slide, why don't some raiders stick with siege equipment and, after it's all done, a branch goes to harass the colony? This would require an improvement to raider AI to run away if aimed at or taking damage, but a group of raiders locking down a section of the map rather than letting you wander out of your base would be pretty neat.