General Discussion

Technology improve, and as a result, we have some big seamless worlds, like Lotro, and  more or less, Vanguard.

But polycounts, texture sizes, also get bigger. A XBox 360 only have like 512 MB of ram, and that is not enough memory to have a "infinite" ammount of textures on screen.  Some people have office computers, with Intel integrated crappy cards, that have even less than 512 MB of vram.

Streaming can able stuff like GTA 4. But streaming is not all powerfull, and limite variety. On a GTA4 area, you will only see a variety of cars and pedestrians.  You can't do that on a MMORPG with player caracters.

So we have more horsepower, more RAM available, but also needs lotsa more RAM and horsepower. A simple solution, for the developers, is created limited zones, and then, remove the need of a strong streaming system.

With a zone, or a intance, the dev's can list the needed textures, maybe 20, and have then already load, always. So is faster for the computer, and easier for the dev.

Games that are already stressing the systems, may have streaming withing a zone. But the streaming here has to be minimal..   but big enough to need a load screen, every time you are killed, and need to move a different subzone.

Also big is not always fun. Is not a tecnical problem, but a social one. Big maps may result on low population, or low density, and dificult to LFG. That could be unfun.

Having instances/zones,  is easy on the server, has these subareas can be handled by different machines on a "farm" of machines. Moving from one area to other, could mean a transparent login to a different machine in the farm. Easy to code.

There can be also easy to debug. You could have a zone shutdown, while a error is fixed, but everything else up, so the players can still play.

Games, that hare giganteous, like EVE online, hare limited already, by the "systems", and you warp from one system to other one. And all systems act as "zones".

IMHO,   zones are soo usefull, make coding soo easy, and solve soo much problems, that are here to stay.  In a perfect world, we will play streamed games (like lotro or gta), but this is not a perfect world.