Of course there are those people who like to see the gameplay early and find out if what they're spending money on is really worth it.
I agree with this point in that it is good to know what you're getting before you purchase it.
But I don't think leaks are a proper or fair way to do that.
If you pre-order a game before any full game-play footage is out, then you have to take the risk of having paid for a game that might not be good.
If you want to find out what the game is like before you purchase it, then do not preorder it and do not buy it on the day of release.
You either take the advertisers word for the game's quality and purchase it immediately (or preorder), or you take a little time after the game has released to read reviews and watch videos reviews and Let's Plays and then you decide to buy the game or not.
Getting a copy early, by whatever means, and then broadcasting any info from it is just massively unfair to both the developers of the game, who have put a lot of work into advertising and creating the game and putting on a big show to it's fans, and to the gamers who are going to buy the game at release and want to experience a new game and all it has to offer by themselves.
I'm happy when a developer released tidbits telling me what their new features are and their progress. I don't want people to give away everything else before I've had a fair and proper chance to purchase and play the game myself.
People seem to think the Game Media is different, but it's no different than books or film.
You'd be happy to see a trailer or preview-chapter from a film or book, but you'd be pissed off if someone came along and gave away the entire plot and everything interesting/new prior to it's release.