The main reason is because at one time we were going to have a system in place that was automatic, a pretty cool guild system -- if only we could have gotten it to work properly. Since it based everything off of character name and bioware id (nwn forces us to use that for uniqueness) we had to make that rule. NOW...I still plan to have a guild system in place and it will still have that requirment (due to nwn restrictions) but sadly we have been swamped with getting other systems in place.
With that said...I am willing to slide on this rule a little but it needs to be in an in-character and RP fashion. But when I do this I need to have the communities support that when I (or the team) state that we do not think it would be an RP or IC reason that we don't get slammed every which way.

Also...when/if (and I really hope we do!!) get the coded system in to play we will need to switch it to where a character can only be in one guild at a time (and get the rewards of the coded system) but they could belong to another (but not get the coded rewards).
Does that make any sense or did I just ramble on?