I'm pretty sure that the reason that Thaos comes in and messes it up regardless of what you do is Obsidian trying to explain what it was like working on their previous games. 

What?

Obsidian makes a novel, entertaining setup, then the publisher comes in and is all "No, don't do any of that, just repeat these cliches because they appeal to the lowest common denominator!"

 

I can see it.

Edited April 18, 201510 yr by Peanuckle

I would argue it's a callback to Neverwinter Nights 2's famous trial scene which in turn seems to be inspired by a similar scene in Chrono Trigger. Basically a chance to review what you have done up to this point, feedback for when you went out of your way to solve quests and an opportunity to show off your character's abilities. Just about every disposition, ability score or skill opens up new dialogue options.