On the dashboard of my car is a speedometer. Also present are a fuel gauge, an engine temperature gauge, an odometer, and a tiny picture of my car that lights up in different places when the corresponding doors are open. Some of these things are legally required to be there, but even if they weren't, I would never buy a car that did not come with them. So why in the heck was I just playing an MMO that has a barely functional threat meter, no dps/hps meter, no system of effectively sifting through its (glitchy as hell) action log, no simple search bar for a character's inventory, no built-in coordinate system in its maps, and no built-in boss timers?
One of the real killers of immersion in World of Warcraft was the fact that the end-game, while technically playable without any addons, really required a suite of third-party downloads to experience effectively. Certain addons were mandatory for certain classes (tanking was impossible at points without a good threat meter), and others were absolutely required for raid leaders. While Blizzard has gotten better about this over the years, the fact that one of two addons can still make a significant difference in a player's numbers is truly damning. Constantly checking the four corners of one's screen because of the ridiculous spread of important information seriously ups the difficulty level of moving out of the fire that you're standing in, and at a certain point (like when it becomes apparent that Power Auras are necessary) beating the addon UI mini-game becomes a mandatory, though not universally fun hurdle.
Even if Bioware does not support addons for SW:TOR from release, they will eventually. Developers aren't perfect, and they'll almost certainly miss some features that players really need. However - if they're smart - they already have a team in place to clean up and incorporate the best and most popular third-party addons into their UI with the utmost of haste. When I start blasting my way through SW:TOR's raids, I want to be able to do it just as well with their default UI as captain "I make my own addons" (that may or may not be a Zabery reference). One hallmark of a great game is letting actual player skill (and my slow, old-guy reflexes) determine just how bad of a player I am, not my inability to set up this newfangled addon.