It could be argued that World of Warcraft has the best user interface possible, because there are so many mods and addons out there to modify it and tweak it to your exact preferences. But curiously allowing addons is not an industry standard. Neither Star Wars: The Old Republic nor Guild Wars 2 have them. And that point was really driven home recently when Bioware was forced to backpedal on a decision to give free players only 1 quickbar in SWTOR. Players obviously felt that having less character slots or banks slot was a reasonable restriction, but being forced to use an inferior UI was not.

Now I'm not saying that mods don't have downsides. Sometimes they provide too much information, leading to social exclusion based on addons like Gearscore or DamageMeter. Sometimes they are simply too good, becoming mandatory for raid content. But in many cases mods that change the UI are just making the game more comfortable for the user.

What do you think? Should MMORPGs all have the possibility to change the user interface with mods and addons, or not?