When I was in a band, everyone would have a couple of ideas for a song before we even got in the same room together. Then we would toss out our ideas to everyone else until we found something we all agreed was worth working on. Once we had a single idea, everyone else would put there own thing to it. Usually we would jam out on the idea and somebody would do something cool and then we would work off that. Like the drummer changes the beat. Well then we would write something to what the drummer did. Or the other guitarist will think up something like, "hey, after we play through this we'll go to this". Alot of brainstorming and tossing ideas until we find something that works, and then we continued on. Quickly it becomes a snowball effect and we get through a song rather quickly. Building alot of the times from a single 10 second idea. I think it's more important to have a good group of people willing to throw out ideas than 1 songwritter. Songwritting in a band is a collective persuit.