The problem is that outside of Venza, none of those major cities are particularly well defined, and most don't even have a name. By at least pinning a name and location to these cities, a lot of the difficulties could be resolved. Once those are defined, than assuming things like trade routes, teleportation ciricles, and other forms of travel and communication can be used quite easily. It is important to remember that not all DMs are going to want to have to come with such a major hub at the same time they are writing up an adventure that doesn't have anything directly to do with such a place. By developing a basic outline of the key spots of the world, we can make that part of the adventure writing much less of an issue.