In this much-referenced technical piece originally published in Game Developer magazine late last year, the team behind idiosyncratic MMO success EVE Online discusses precisely why sharing a single world between all of its players makes sense.
Most of the larger massively multiplayer online games use separate instances, or shards, of the game’s universe in order to manage player populations and server issues. We feel that a single shard should be the natural choice of any MMO developer, and that’s what we do with EVE Online.
When you ask the question “Why a single-sharded architecture?” it’s also informative to look at the deeper question: “Why have shards?” There are two main reasons why a developer chooses a sharded implementation of a game — lack of content and technical challenges. These are actually inter-related.