Maybe they only felt the need to explain Jake's right to be in the game, to make him more important and interesting instead of having everyone wonder: "Who the hell is this guy, why is he in the game and not Jill or Claire?" As for my part, although I think they do spoil us pretty much, the revelation of Jake's identity made me want to play this game even more. Since the release of that second trailer I've been looking forward to Jake's meeting with Chris, and how Chris will react once he finds out who the guy really is... among other things.
So, in short: I find it important to release some story spoilers, but seeing what Capcom is doing at the moment, I agree with those of you who say they're already giving away too much. However, until the game is out, we can only speculate on how much they're actually giving away. I mean, although we know that Jake is said to be Wesker's son, we still don't know anything about the circumstances of his birth, who his mother is, if he was conceived in a natural way... or - SURPRISE! - if he even is Wesker's son. It might turn out to be a big lie after all, especially when you consider who tells Jake this alleged truth. We are still left in the dark concerning these matters.