As long as you don't call it "Diplomancer", feel free.

After some further thought, I think I'll add an option to auto-update your default faction based on which faction's championing tabard you're wearing. Will that work for you?

My experience with Squire/Squire2 has been the opposite. I referred numerous people to Squire, and none of them were ever confused by how to use it. Typing "/squire" to bring up the options panel was intuitive for anyone who's used an addon, and "drag the macro onto an action button" was all the explanation I ever had to give. Once Cataclysm hit and everyone had to switch to Squire2, I've had a good portion of those people complain about the moved/changed options, and even watched several of them in real life as they struggled to find the new options, and squint at the checkboxes overlaid on icons. Even once they found the checkboxes, most of them didn't notice the button at the top of the window leading to the rest of the options.

As for XPBarNone, the internal code being run when you right-click on the bar and select a faction is exactly the same thing that happens when you check the box in the deafult UI, which is also exactly the same thing that happens when Diplomancer changes the watched faction. It would be possible to differentiate between "watched faction was changed by Diplomancer" and "watched faction was changed by the user or another addon", but again, I'm not interested in adding a bunch of bloated flags and checks and extra event processing to do it. Using the options panel to change the default faction is pretty quick and easy to do. How often do you really need to change your default faction anyway?

You can also set the watched faction through slash commands:

/dm default earthen

It's not case sensitive, and doesn't require the full faction name, just enough of it to uniquely identify it from other faction names.

UPDATE_FACTION is an event that fires whenver the player gains or loses reputation with any faction. It has nothing to do with setting or detecting the watched faction.

It would be possible to hook the OnClick handlers for all of the "Show as Experience Bar" checkboxes and assume that if a user is clicking one, they want to set that as their default faction, but this is not something I will ever put in Diplomancer, for several reasons.

Firstly, all of the other options related to Diplomancer's behavior are in the Diplomancer options panel. I see no reason to split them up.

Secondly, though it seems to be all the rage these days, I don't see any intrinsic value in having an addon's options integrated into existing Blizzard UI panels versus having them in a dedicated panel in the Interface Options window. I use a number of addons that do this, and with most of them, it feels very clumsy, breaks or unintuitively modifies default UI functionality (which is what would happen if I implemented your suggestion in Diplomancer), sacrifices functionality that was present in an earlier version of the addon that used a dedicated options panel, and/or makes the option controls hard to see and use (especially for people like myself with poor vision) since they're shoehorned into a layout that definitely wasn't written to accommodate them.

It's also annoying to install a new addon and have to spend time guessing which window its options might be linked to, instead of just being able to open the Interface Options window and have them right there with all my other addon options. Having an addon's options split up in different locations is even worse.

For example, the addon I use to provide a "travel form/random mount/dismount" button recently made this switch. Previously, the addon had a dedicated panel for all of its options, including the ones to let me select which mounts I wanted it to include in the random selection. In the panel, mounts were grouped by type, so all the ground mounts were listed together, all the flying mounts were listed together, etc. Selection was easy: click on a mount to toggle it on or off. Selected mounts were shown in color with their icon higlighted. Unselected mounts were grayed out and not highlighted. You could also shift-click to quickly enable or disable all of the mounts in the category. After the switch, mount selection was done by means of checkboxes overlaid on the mount icons in the Blizzard mount UI. This was a huge downgrade. It's hard to see which ones are checked; on some icons, it's hard to see the checkbox at all. There's no way to quickly enable or disable whole categories. I have a lot of mounts, which means I have to click through a lot of pages to find a particular mount to enable or disable. There's no division by mount type. Nothing was gained by making this change, and a great deal of functionality and usability had to be thrown out to achieve it.

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Re: Cataclysm — Yes. However, I've had very little time to play at all, and between that and the 3000+ queues for my realm that take 3–4 hours to get through, I've only played through Vashj'ir and most of Deepholm so far. If you want an update to happen faster, feel free to post lists of which factions should be associated with which zones or subzones.

Re: Default faction — No, it can't. Checking the "Show as Experience Bar" box is how you select a faction to watch. When Diplomancer changes your watched faction, it is doing exactly the same thing "behind the scenes" that the default UI does when you check the box for a particular faction, effectively checking the box for a different faction, causing the box you'd previously checked to be unchecked.