For one thing, people wouldn't stop posting spoilers in thread titles, most people wouldn't even conform to using "[Spoilers]" or "[No Spoilers]" tags in threads so that when people who see thread links on the front page or in their "New posts since the last time you were here" page, know that when they see an innocent looking thread titled "Shinji is the best!" they know that it's in the rebuild subforum and that it actually contains spoilers galore. The worst was thread titles that contain blatant spoilers because those are exposed by default in new post streams. The thread titling has actually been mitigated this time because of the nazi on-topic rules that were employed when the forum was so loaded that moderators couldn't rename threads, move posts, lock threads, etc. because the moderator control panel usually takes twice as long to load than viewing a thread on a good day. (Note: the "[Spoilers]" tagging was tossed since nobody followed it, with warnings and all, and the entire subforum became fair game spoilers except for the 2 or 3 threads with the "[No Spoilers]" tag on it).
Second, as soon as production images or new trailers would come out (that would fall under the spoiler policy because they were released after the movie) people scurried to make avatars out of them. You'd think it would be a no brainer not to put stuff like that in an avatar, but people did it in droves. Admittedly, most of them were borderline but some were pretty spoilery, Mari nearly "kissing" Shinji was a popular one.
Finally, it didn't prevent spoilers from staying in the rebuild subforum. While discussion about rebuild stayed in the subforum, you had plenty of "but in Rebuild you could see Bardiel in the plug with Asuka" for example, in the Discussion Forum thread about whether Bardiel needed the entry plug or not, or Bagheera's comment about 3.0 Shinji's situation (no spoiler tagged, no warnings), completely a remark in passing in the pictures thread.
As an aside to the 3 points above, I'd like to note that the policy started with a "keep spoilers in the rebuild subforum" and when each of these issues arised, we needed to amend the policy to account for them.
As for #3, the number of people who left during 2.0 (there's a thread full of them, which doesn't include the PMs and emails that I got from others) and it's year long spoiler policy, was probably 3 or 4 times more than everyone complaining about it in this thread combined. Some of these people felt they had to leave the entire forum because spoilers were flying around, unmitigated by warnings, in thread titles, avatars, or just plainly out in the open for all to see in threads outside of the rebuild subforum. Some claimed that they'd be monitoring the website's front page for news on when it is ok to come out of hiding, since like some here have correctly noted, the rest of the internet is pretty much "we don't care about spoilers, so if you do, then leave now" and there would be no other way for them to know if there was a Japanese DVD release of the movie or when an announcement is made. That's one of the main reasons we have a spoiler policy at all, because if it's just left to #1, we may as well not bother with spoilers and let the couple of dozen people who really want to stay unspoilered leave the forum for a year.
It's probably in response to the absurd level of hyperbole from the people who feel their rights as forum members are being trampled. Eventhough this policy was used for 2.0 for a better part of a year, and is even less restrictive than the 2.0 one, and nobody complained about it back then.