Raiders units are to me a very interesting unit: mediocre auto units which fires and somewhat helps the attacks but at a cost (destroying your possible future buildings, like sacking/exterminating), and sometimes not very efficiently (because not controllable).
What I really dislike is the idea that, as you said, fortifications just fell "after a time" and then its a complete street fight: barricades should somewhat balance the issue of having nearly useless walls but I really have poor hope on how well CA will balance everything together for the unmodded release version.
I dearly hope it wont become the only way to go through siege battles... many have complained of "siege AI" and "too many settlement fights" but the solution CA offers seems like exchanging an evil for another. Will need to test the actual game before being definite on the issue.