Yeah, I guess that's one of the reasons why they did it: to calm down the drama before it happened. Still, I wish they had kept it a secret until the show aired. We rarely ever get events of this proportion, and the one time we do it gets spoiled for almost everyone. It seems that you don't mind spoilers much, but keep in mind that there are many people out there who really hate them. I for one really dislike them when it's a show that I care about. Starting a show late doesn't necessarily mean you will get spoiled no matter what. If you take the proper precautions, you can elude spoilers completely no matter how late you start a show. There have been a few shows I was a little late too, and I was still able to avoid spoilers. Just don't go on sites related to the show or search up scenes on YouTube. There is always the off chance that you'll stumble upon a spoiler somewhere on the Internet, but in those instances there's nothing you can do really. I like to avoid spoilers, but I'm not going to avoid contact with everybody and not go on the Internet until I'm caught up.
Also, that whole waiting thing isn't really fair when you get into the show later. Let's say a prospective brony just learns about the show now, and comes across Alicorn Twi before he's caught up. It's not that he waited too long, he simply learned about the show late, and that's not really his fault at all. It's a shame that so many future fans will not be able to truly see Twilight as a Unicorn. As they watch through the episodes pre-s3 finale, they'll just keep waiting for Twilight to turn into an Alicorn already and not be able to experience the show in the same way that we earlier bronies did.
What you said about expecting people to keep hush hush about spoilers, I agree with you. Given enough time, talking about spoilers publicly is something that's just inevitable. I'm not blaming the fans; I'm blaming the decision to turn Twilight into an Alicorn. The people behind the idea of Alicorn Twi probably knew that this change would be hard to avoid for future fans, but they don't really care I guess.