In some ways, yes, but in others, it helps keep order. I prefer it that way. If you create a thread, you get full control, in the other way, someone could - in theory - land a spaceship of aliens in a fantasy world and run roughshod over them. 

Personally, I prefer them to have a singular focus. Fantasy all the way through. Modern All the way through. Sci Fi all the way through. It helps the ease of play because you aren't haven't to create different checks and balances.