Also because the development of an MMO takes so long and things tend to change from time to time, whole systems they find not to work will get re-done, concepts get changed. It would simply be a disaster for a dev if they came out with everything only to have it change or be removed during the dev process. You complain now for lack of info but I guarantee it would be 1000% times worse if you publicly saw things change drastically over the course of development like they do.
So yeah a slow trickle and only giving info when its 100% going to be in game is the wisest choice.
Yes, but on the flip-side you don't ever let fan - let alone journalist - opinion turn indifferent or downright sour. Almost every game site rated TESO below a number of upcoming MMOs in their post-E3 wrap-ups, which is a bit ridiculous considering the fact that TESO is undeniably the biggest franchise with the biggest budget and the longest development time. But instead, everyone wants to hear about Planetside 2, Defiance as even Neverwinter. However you cut it, TESO just isn't getting the sort of media traction it needs.
Everyone here knows about hype, we're not new to the scene; but people can also remember that Skyrim's unveiling and subsequent E3 demo got people excited - it had the press talking. That's how hype works, from the get-go. If they're going to wait until 3 months out of beta to announce that one big feature they've been keeping close to their chests, then they're going to have sacrificed a lot of good opinion that they won't be able to get back so easily. And considering the lengthy, spolier-heavy nature of MMO betas, you're better off getting people damn excited and then drying out a bit during beta because that will happen anyway, rather than having the thing be dead on arrival.