I think if the story is compelling enough, it doesn't need some earth shattering repercussion for tarrying to long taking in the scenery or, you know, playing the game. If the main quest is a personal one, at least initially , as it is in Planescape and New Vegas, it gives the player a chance to get out and experience the world, talk to the NPCs, do some sidequests, without feeling like they should be rushing off to stop bad stuff from happening. I think it also holds the story together a little better because you're going to help Farmer Joe find his missing chicken whether the demon lord is going to torture your kidnapped mum and then destroy the world or not. If the player is off doing silly little side quests, it tends to make the threat of impending doom, whatever it might be, a little toothless.