By the way, there are guilds around which don't like it if you've alts in other guilds. Or at least want to know about it. Of course they wouldn't know it if you don't tell them. And if one would separate IC and OOC for 100%, it won't give problems. That's theory though, in practice people rarely reach that 100% .
Plus there is the griefing related with the bad practice of alt-espionage, and as making new accounts is trivial, anybody could make a somewhat effective, but completely OOC, Intelligence Agency, provided one has enough alts inside several guilds. Of course such action wouldn't be well seen by the community, besides being arguably a form of cheating and perhaps something that deserves a ban if caught.
Just think about it: You have an alt on guild X that is an enemy or at least rival of the guild Y where your main character is. Then through your alt you discover something really important that could be used against guild X. "Playing dumb" not only is difficult, but also isn't fun at all.