If every MMO HAD to copy one thing from WoW then the addon customization should be it.

• It adds another layer to the existing game that you have to pay relativly little for
• You have your customers work for you... for free
• You get the collective creativity and imagination of all the trekkies... seriously, if you could harness this we'd have achieved lightspeed by now.
• You take the strain off your UI design team - Let the players make their own UIs and maybe institue some of the better ones as options in the official UI
• You increase the community, personalization, and attachement that people feel for the game.
• You still have 100% control over what goes in or comes out of the game. Don't like a mod? Ban the function.
• People really want options, it's even one of your business designs. Give them the ability to create their own options.

I mean even a limited UI modding sceen would make a lot of people very happy. The first 3 mods I see everyone making are:

1. Taking out all the random crap from Sector Space
2. Removing the shield indicators around the ships
3. Combining the mission screen and the map screen for easier travel

Do those really sound too bad?