Do you remember Obsidian doing something similar ever in their games?
Yes, Alpha Protocol.
Where? It was dialog impacting gameplay and not the other way round. Ok there was that one, single instance when if you managed to finish the level with stealth you got more data... which weren't really that much useful for you. Nice job - one flavour shit that's useless to everyone - so it wasn't even well done.
Also the example of Alpha Protocol only strengthens my claim that their games are lacking and not thought-through. How about an example of gameplay mechanics working and not sucking arse (e.g. compare AP stealth to Mark of the Ninja - a platformer stealth game. Sorry to say but MotN wins - there's no contest).
One more thing: this time around they are going to have all that excelent quality story with tactical RTwP combat. I am really wondering how they are going to combine water and fire, because they haven't really shown they can accomplish that yet.