I have found it better to have stronger scope for 2nd and 3rd iteration feature sets in mind.. refactoring because you didn't think you'd be adding a certain kind of feature or filter or database scope is worse than knowing ahead of time that's where you were going.

A little different than "spec", but one-shotting things works great if that's going to get you as far as you want to go. If you're adding neat-o features after that, it can get a little messy because the initial design doesn't bend to the new idea.

Even something like adding anti-DDOS libraries towards the end, and then having to scope those down from admin features. Much better to spec that stuff at the outset.