IMO, the storyline is the basis of a mod. It basically plans the rest of the mod for you, if its good. It also makes you visualise the mod itsself and gives insight into the different teams and what abilites they have.
I spent about 2months on the planning of my mod for the storyline, as soon as i finished it's easier to think of units to make.
Read mine if you want:
http://online.deezire.net/modules.ph...iewtopic&t=700