Imagine you are the president of your local garden gnome painting club. The club meets regularly for sessions where the members sit together and paint garden gnomes. Every year there is a charity event, with the painted garden gnomes being sold for charity after an exposition. Now your garden gnome painting club has lost some members who quit the hobby or moved away, and you are interviewing potential new candidates. What do you look for?
Given the club's activities, it is obvious that what you'll be looking for is people who are nice and friendly, and get along well with the others during the regular painting sessions. Being dedicated and turning up regularly is certainly a plus. But how skilled the candidate is at actually painting garden gnomes is only of secondary importance. The club is there to do things together, to share a hobby, not really to overcome some specific challenge.
Now lets move to another example: This time you are the coach of a high-class soccer club, like Manchester United, looking for players you could buy from another club. What would you be looking for in that case? Obviously the selection criteria a very different. The soccer club needs to win games, and can only put 11 players on the turf. Thus soccer skill is of utmost importance. A minimum of teamwork is required, but you'd rather take a prima donna playing great soccer than a nice guy with mediocre skills.
And now to move from those examples to the world of MMORPG: Is a MMORPG guild more like a garden gnome painting club, or is it more like a soccer club?
The first guild I was in for a prolonged time was in Everquest. In Everquest it took 2,000 hours to reach the level cap, and most of the players I was guilded with didn't count on ever arriving there. Thus there was no raiding at all for us, just playing together (soloing was hard in EQ), chatting, having fun. It was definitely a garden gnome painting club. During my years in World of Warcraft I was for a time member of a hardcore raiding guild (I got up to Nefarian in BWL with them), and that was more like a soccer club. They didn't invite me into the guild because I'm such a nice person, but because they needed a holy priest, and had in a test raid seen that I was performing my job well. When I took a break from WoW, I was immediately kicked from the guild.
Where MMORPGs have a problem is that you can't be a member of both the garden gnome paining club *and* the soccer club. Guilds often try to be both, and then frictions arrive between the two parts, leading to guild drama. As the examples showed, already the hiring requirements are very different, and while there is a certain overlap in activities (run heroics to have fun vs. run heroics to gear up), the ultimate purpose is a very different one, and at some point the needs of the two sides diverge. You can work around that limitation by having a main in a raiding guild, and an alt in a social guild, but even that is often frowned upon by the raiding guild.
What would be a lot better is if your social network had a different structure from your "professional" association in the MMORPG. Instead of having guilds, a game would have one social network structure for people to hang out with together, and a goal-oriented structure for people to raid together. Those wouldn't even need the same tools, the raiding structure might have tools like a raid calendar or DKP system that would be useless for the social network structure. Instead of having to switch to an alt in a different guild to hang out with your friends, you could be a member of both simultaneously. The big advantage would be that your network of friends would remain unchanged even if you decided for some reason to quit your raid structure to join a different one more appropriate to your dedication and skill.
In short, I think that guilds are trying to combine two functions that aren't always all that compatible with each other, and we would be better served if these two functions were split up into two independant structures.