Well, my take on it is Larian is trying to make an immersive game where your actions have actual consequences. One way to show this NPC's disappear when killed or get affected as a result of a decision of a quest. Also bands of Orcs and other creatures don't respawn when you have cleared an area or affected an area due to the way you solved a quest.
Also a poster above me nailed it when he mentioned exp and character trait game mechanic limitations.
If they did have respawns and you could grind your char to max lvl and gear before you were 1/4 through the game, that would kind of ruin a lot of the game for the other 3/4th of the game.
Also of note are the mods. Yes you may beat the game and not have any more interest on revisiting the main campaign, but mods are free and keep you busy for quite some time.
I have been itching for a game like this where the world is dynamic and actually reflects my actions and deeds. I was hoping Skyrim would have been that game, but sadly not so much.
End point is I think you will certainly get your monies worth.
might interest you. The campaign has ended but I am eagerly anticipating the release (didn't back high enough for alpha access). Well, assuming you didn't come accross it already, of course
