@Tor IMO there's two major ways to look at guilds: 1, A form of Content; 2, A Community.
1, Obviously, we're talking raids, dungeons, mass-pvp, whatever. That sort of extra IG content. In my own experience, and opinion, guilds that focus just on this aspect of gaming are doomed to fail.
2, When you start doing that content, would you rather do it with randos, or friends? Also, what will you do, when you have maxed out gear? Stop playing? What was that playtime for? If you just play and hang with friends, there's any number of content to be had, especially in a Dynamic MMO. You can roleplay, you can create lore, build cities (even past the point of minmaxing), do the same IG content over and over again, as well as make your own content. Create events, for example.
So, well, what I'm trying to say is, that both ways of playing are cool, 1 is more suited for lone-wolves, with the mindset of clearing the game, while the second is for social players.
I personally have plans way past the point of 'clearing content' with our guild.