The problem is, design is not something you just slap on later. If it is not here from the start it won't be there at the end unless you redo everything from scratch.
Design as an afterthought is a lipstick on a pig.
I don't think design should ever be slapped on. But I do think it makes the most sense to test and launch a first version with what amounts to a functional wireframe/prototype, if possible.
