While we wait for CCP's comment on the current player-outrage within the Eve community (more on this soon, I hope) we could pick over this recently-posted article on Gamasutra, in which one of Eve's key architects, Dr Kjartan Emilsson, discusses the benefit, technical challenges, loopholes, and programming horror of a single-shard MMO.
Such a player-driven system doesn't strictly require a single shard to function, but it is catalyzed by the extended size inherent to single-sharding. A small economy will be manipulated by a few strong players and exposed to large fluctuations and instabilities. The larger the economy gets, the more resilient it becomes. Once beyond those instabilities, it truly starts to reflect the macro-economic landscape of the game-world, becoming an all-pervading, autonomous, and ever-changing mass of social content that no designer could ever think of hand-crafting.
It's interesting to see this addressed from a technical and design standpoint, but also interesting because it is such an unusual approach. I wish we saw more such epic undertakings within the MMOs. Go read.