I think it would be MORE worth it for a short trip than a long one. You probably won't mind planning where to eat for those two days (to maximize your savings) and will be able to eat it all. For long trips, by day 4 your appetite will likely go down hill and you will be skipping desserts, or snacks etc. Disney counts on those unused credits -- don't let them have 'em!
For a short stay of two nights, I'd even consider the Deluxe plan as I could stretch the credits into the third day and eat all Table Service meals, perhaps using my snack credits at breakfast.