I don't see any harm in having stretch goals, if backers are going to buy extras or new backers take an interest after seeing the trailer then it doesn't hurt to have goals for further development. If we hit them then it's awesome and we get shiny new toys, if not then we get what we originally expected (which after the trailer I'm really pumped about).
On a side note I haven't browsed the forums in a while but how many companions and how large a wilderness is already planned for Pillars of Eternity?