Before I began playing MMOs on a regular basis, I played a lot first-person shooters. During that time I became involved with several clans with the dream of one day being a professional FPS gamer. That dream never came true, but I did learn to appreciate the work and camaraderie of being a part of a clan. When I started playing MMOs I immediately got involved with a guild because, unlike most FPS games, MMOs encourage players to work group together.
Having been a guild member and a guild leader I know that guilds are formed for various reasons and there is a guild for every kind of player. It is up the guild leaders to determine if their guild is for socializing, casual leveling, focusing on gameplay such as PvP, or accomplishing specific goals in the game. The first guild I belonged to in World of Warcraft slowly progressed into a raiding guild which was a trial by fire education for me as I learned the importance of character stats and boss fights. There came a time where raiding was not as enjoyable as it had been, so I left to start my own guild. Because of my past experience I chose to form a guild with the sole purpose of helping lower level characters which filled a need on the server I was playing. I enjoyed playing WoW a lot more at that point because I was able to cultivate a small community of casual friendly players.
MMO developers have improved the tools to help manage guild activities and introduced incentives to get guilds to work together. Guild advancement systems are now common place and have changed how guilds function. Guilds that work together are rewarded with perks such as experience or gold boosts. This has turned guild member recruitment into a two-way street. It used to be that guilds, with the exception of hardcore raiding guilds, would take any player, who would ask to join, to build up numbers. But now guild leaders have some leverage by being able to offer the bonuses that the guild has earned to potential guild members. I was recently interviewed by a guild leader in Star Wars the Old Republic when I asked if my alternate character could join the guild. The guilds in SWTOR are able to provide up to a 10% experience boost based on the number of guild members they have. Guild advancement systems are not perfect, for instance I do not like how they benefit larger guilds as well as act as a carrot on a stick to keep players actively working on old game content.
Belonging to a guild continues to be an integral aspect of playing MMOs. In fact one of the reasons I am now playing Star Trek Online again is because when I returned to STO after it went free-to-play I was surprised to find my character still belonged to a guild. Later that same day I was invited to join other guild members to complete some fleet actions, it felt really good to be included by the guild. I will spend time playing a game if I am actively involved in guild activities.
When Guild Wars 2 was in development the team responsible for guilds must have felt that if players are involved with more guilds they will spend more time playing the game. Every player account in GW2 can be attached to several guilds at the same time. Unfortunately the guild system in GW2 has contributed to my diminishing play time because I am not connected to a single guild in which to focus my efforts. Having the ability to switch guilds on the fly may appeal to some players, but I do not see how that helps foster a unified guild community.