As far as I know, the reason most big MMOs have multiple 'worlds' is partly because they have many thousands of players. As their worlds are not procedurally generated, making enough land to keep things interesting for such numbers would be a nightmare, hence it is easier to have multiple copies of the same world. Similarly, bandwidth issues caused by such numbers mean that the servers for those worlds are strategically located to cater to different regions.
Frankly, I don't see H&H requiring multiple worlds, for several reasons:
Firstly, even if, in the long run, several thousand players did end up on H&H (and I hope that does happen!), from a design perspective this is not an issue. J&L are working on the concept of a world that is procedurally generated to be elastic. As long as there are hard resources (memory/bandwidth) available, then there will be land. Moreover, the concentric design means that it takes more and more players to keep pushing out the size, as each ring encompasses more land than the previous one. If we had double the number of rings, say, as the current world has, the total land size would be more than tripled I think (might want to check me on this, I've done no calculations).
Secondly, I might be wrong, but I don't see why J&L would want to set up different worlds solely for the purpose of catering to different players gaming desires. As a sand-box style game, it is largely up to the players to decide how things are going to be. Of course, you could have a world where PK is impossible (sounds boring to me), etc., but if people want to play a different type of game, they can just go elsewhere. Unless/until J&L want to start making H&H a profitable business, there is no need to provide multiple visions in order to maximize player numbers.
Yes. Yes this is a fertile land and we will thrive. We will rule over all this land! And we will call it... this land!