Anyway, there are some UIs out there that are hugely cluttered. There are also some UI makers that make it VERY clean. WoW let's you essentially change everything to your liking via addons. It was built around the usage of addons.
Some people have had a bad experience with addons and it left a bad taste in their mouth. Idiots abusing Gearscore and other such addons to "segregate" the wow population into casuals, elites, etc. With how GW2 is being present we don't need, and obviously quite a few don't want, this to happen. So they feel that the easiest way of keeping this from happening is to keep such addons (even though they can be immensely powerful tools) from GW2. Will it stop people from lumping everyone into groups? Probably not.
Truthfully it seems to me that ANet thinks that all you to play a game is your eyeballs, fingers and ability to take in the information on your screen and adjust accordingly. You shouldn't need an addon to tell you anything.
Are there going to be addons in GW2? Probably. Why? Because despite ANet's no support of addons in the first one, people still decided to make addons. They were, mostly, simple UI modifications. Made the sins look shinier or sleeker. Personally I think that this is going to be the kind of addon we see for GW2, and really one of the few. I think this because even though GW2 is going to see people from all kinds of mmos and games, I feel the vast majority will be GW1 players and it seemed to me a lot of them never felt the need for any fancy addon like recount. I also think ANet did a good job of making a clean, simple but beautiful UI.
Even if addons like DBM or Recount or whatever do make an appearance in GW2 I doubt I'll have them. It just seems like it'll be a lot more fun to reply on my own reflexes and judgements (even if they pretty shitty) than staring at addons to see when is the next Meteor Storm coming so I can get ready to move five steps over and then stop again and faceroll my keyboard.