Tipoffs and word-of-mouth could work for some espianage type activities against other guilds, but it is a lot easier to believe that some one did something when you see them(or someone that is disguised as them) do it, rather than hearing, \"oh, so-and-so did something. You\'ld better get him/her, and his/her guild as well!\"
Another worry of mine, is secret charecters. If someone from one guild, makes a seccond charecter, and gets it into another guild, then breaks it up from the inside, or uses that charecter to gain IDs, passwords, secrets, etc. It always bothered me when people did that, but there isn\'t much that can be done to stop it. Guild leaders just have to be intelligent enough to spot shady charecters.