We aren't talking about serving up web pages, or single-player experiences. Each time you visit a website a different server in a load balancing setup can be used because the content is the same. But in an MMO, other players are part of the content.
Friendships exist outside of just guilds. Players form friendships with players in other guilds, players form friendships in real life and then want to play with those people, etc. This doesn't address the fundamental problem that most players want to be where everyone else is. You're going to have players abusing the system to get the desired effect: everyone joins one giant guild so they can all be on the same server all the time, etc. If you give the players any amount of control, they will use it to do damage. Maybe not with malicious intent, but it'll happen. This is why some WoW realms are overcrowded, and others are ghost towns.
Everyone wants to be able to interact with everyone else seamlessly. Anything less is a detriment to gameplay.