They are more of social groups of friends that compete for glory. Hey, why not join Fellowship of Chocolate cookies and be a part of one? We treat you like family (Except without all the hugs and kisses).
Also, it's a lot easier to ask people for advice or ideas if you're part of a guild than just using the main message boards (which can be good too). Plus if you take a look at the events list, you'll see that a lot of them are guild created ones (or are open ones but have lots of guild mates in them).
Oh and let me plug http://www.urban-rivals.com/guild/?id_The Casual Guild while I'm here (I just started it )
Over my years of online gaming i've encountered many guilds (or guild-type teams). Almost all are ruined by people's egos. It's very rare to find one that isn't, especially given the competitive nature of this game. Do the potential advantages outweigh the potential disadvantages? Once the game is learnt, how much of an advantage is it being in a guild?
Tag team UR? Yes, that could work, or a system based on the deathmatches for instance. Teams of 4 with the win going to the team that accumulates the most points.