Okay, the title sucks. Sue me, it's early and I didn't get much sleep. But seriously. What's the point of guilds anymore besides storage space for bank alts? I mean, with the rise of cross-realm grouping and the loss of a few key perks (Have Group, Will Travel, for example?) that came with the larger guilds makes me think that perhaps guilds are losing their functionality.
I mean, let's take a look at the history of WoW guilds throughout the game. In Vanilla, guilds were generally a place where you stayed. Guild-hopping was looked down upon, in much the same way as a resumé with too many jobs in a short amount of time would be viewed with uncertainty. Getting gkicked was effectively a death sentence on your server.
During BC, raiding dropped from 40-man to 10- and 25-, causing a little strain on guilds, but ultimately leaving them alone. Slightly less emphasis was placed on the length of time a player was with a guild. BC was a fine time.
In WotLK, guild-hopping became a very normal thing. With the rise of the random dungeon finder, guilds weren't nearly as important because you no longer needed to have a group of people to call for dungeons. Raiding lockouts being tuned to both 10 and 25 players meant that guilds no longer had to be able to field a large roster. Many smaller guilds were born during the Wrath days.
Cataclysm attempted to bring back the importance of guilds, and for a time, it worked. Your guild's level and earned achievements gave you access to more and better rewards and perks, and you had to earn reputation with the guild in order to actually access those rewards. That meant staying with a single guild while you did quests, levelled, and ran dungeons and raids with the guild. That last bit, I think, made guilds a much higher priority during Cataclysm.
However, now that Mists of Pandaria is days away, we can see that some of the changes to how guilds work have made them pretty much unimportant. Guilds are no longer capped on the amount of XP they can earn in a day. Reputation gains are no longer capped, and have been adjusted so that you can go from neutral to exalted with a guild between levels 1 and 30. As such, guild reputation now requires only hours of work instead of weeks (there was a weekly reputation cap, remember?). Guilds can hit level 25 pretty quickly now, and with so many level-capped guilds out there, perks are no longer a factor when deciding which guild to join.
Add in the fact that cross-realm raiding has made it possible to join up with organized and highly-skilled raids without being bound by the restrictions on your server, and you've got no really compelling reason for guilds to even exist, save as social groups.
Seriously, I haven't been awake this early in forever. Time for some coffee.