The idea of not releasing a date was to not give up people's hopes if they can't make the date. If they give a month and dont' make it by then, will you ask again? I'm sure that they'll advertise the release date of part 4 as an accurate release date closer to the time. We wanted it to be released when it was ready- not by a set date. If we want that, then we can't get approx months.
The main issue here is not the specific date, it's communication. People need to know there's a set goal, something to look forward to. Afaik, originally, the release was set to May (at least that's what I believe I read here, and on Wikipedia). That came and went, and from the looks of it a release in June doesn't seem likely as well. With these semi official dates coming and going and the lack of any other set date, people may eventually despair and lose interest.
If you set an approximate date, you actually give yourself more "quiet" time to develop the game without constant, annoying questions (kinda like this thread

). The way to mitigate slip ups in dates is, again, through communication: keep people constantly informed of the progress in high level, and of the difficulties involved. If people know you have a goal, and that you can't, objectively, finish the job in a satisfactory way by the original date, they'll be much more forgiving if you change it, because they'll feel they're part of the process.