Core data is great for quickly putting together models, but it is also really good at letting you extend your models, during development. You know, so that we can actually do the Agile part of Agile Development. Especially if you just want to add or delete a few fields or a few entities, it makes modifications quite straight forward.
One thing that I missed in XCode 4 was the ability to add a new attribute to an entity, and then right-click for the contextual menu to copy both its property declaration and core data accessors for the header, and less importantly, the dynamic implementations for the impl file. You could even grab them as Objective-C 1.0 or 2.0.
They are still there, but somewhat hidden. You still copy as before, but now, go to the header file of your NSManagedObject entity, place your cursor
BTW, the model editor is still pretty buggy. One thing that seems to happen consistently is that relationships disappear, in map mode. Just try opening and closing the Attributes disclosure triangle to fix it.