Also you're talking like they said this as an afterthought. Josh sawyer said they specifically designed the game from the beginning to be able to do a pacifist playthrough. That was something on the plate from day one. Not just something they thought of six months into the development.
Point conceded, if they managed to do this without making the story lame I will be very impressed.
The crux of your rebuttal is still "because the devs said so".
And you overestimate the freedom you will be given. It goes like this
Quest->Quest->Quest->Plot Twist->Quest->Ending
Or should I say in FNV it will be like this... NCR= New California Republic CL= Ceasar's Legion
NCRQuest->NCRQuest->NCRQuest->Plot Twist->NCRQuest->Ending
CLQuest->CLQuest->CLQuest->Plot Twist->CLQuest->Ending
Its grossly oversimplified but it will have to do. You may have leeway and freedom to decide HOW you do things, but not WHAT you do. And you will most likely HAVE to KILL some things, because the next step in the storyline depends on you killing it.
Loghain was a bad guy but he wasn't the MAIN antagonist. Eden blows himself up after you stop him, for all intensive purposes you led to his death, and IRRC you have to kill colonel autumn to get to the GECK in the last scene. Unless I am sorely mistaken there is no dialoge he is hostile on sight.