I happen to think that having a spec is useful even in the very beginning...
and during development...
and testing...
heck even when the project is running out in the wild having the spec available could be useful.
What I'm trying to say is, the spec is always useful.
What I wanted to say is that, sometimes you do not know what exactly your idea is, thus you do not have anything to write upfront. Let your idea to start, like a bit of coding and the spec along with it. It will not only help in planning better but it will give enough time to switch between spec and development. Whenever you are stuck in development take your time, write spec. That's what I usually do.
I see