I'm running a D&D campaign in a slowly developing world. It's a magic heavy campaign as we use the D&D 4th edition rules with my own extra rules incorporating more magic potions/drugs/enhancers and my own version of the runelord magic (Runelord by David Farland http://www.runelords.com/ ). I've only thought about the basic ideas for the world yet. As I've started the players out in a city (which I'm currently mapping) most of the stories have centered around it. They have begun on level 1 so they are not very strong yet (level 4 now) - so, the stories and encounters I have made for them has been week ones, and ones making them friends in a bit higher places - gently guiding them to the storyline I've roughed out. So, I haven't had use for a big world history yet, only some general pointers of where I wanted them to go (now and later) and what type of world they live in. It works fine and I can add to the story as I go along. As a matter of fact I just added a new player to the group lately and I used a random event the group had experienced (being passed by forcehorses on the road to adventure) to hitch up to the back story of the new player.
So all in all - I don't find any reason to plan too much - its easier to change course if everything isn't carved in stone