If your guild is like mine, it's easy to have trouble keeping track of who plays which characters. The guys and girls in our guild play quite a bit, so everyone is usually working on an alt or two, and it's really helpful to know that that 23 mage on right now is actually a well-geared 70 warrior who can tank your instance. But in the standard interface, there's no real way to know who's who-- either you have to learn who plays which alts over time (which leaves you stuck not always knowing exactly who just said hi to you), or you have to come up with a makeshift plan. My guild sticks the main's name in the alt's guild notes, and while sometimes that gets messed up (because an unsuspecting officer changes a note), it usually works OK.
But still, I'd like to see an official interface for it. Now, some players probably use alts as masks to hide behind, which is fine (I have a few secret alts of my own), but if your alt is in the guild, your guild has a right to know the other guild members you play. And an official interface would make it easy-- it would get entered automatically and be easily accessible for everyone. As long as we're working on guild banks and other guild-friendly features, I'd like to see an alt-tracking interface (for just within the guild-- I'm sure that if the same thing hit the Armory, lots of players would cry foul) as well.