They don't suck 100 percent because as a player I do want to know more about new product, but, without sounding like a Wizards parrot, they suck pretty close to 100 percent. Here is why:
1. I know I am being sold an entertainment product. Wizards is not selling me stocks or bonds, or any other financial instrument. I crack their packs, I play the cards, repeat. Its OK if Wizards hypes their product. Dragon's Maze and Battle for Zendikar are powered down sets, and I enjoyed getting hyped about both sets just as much as I enjoyed getting hyped about Origins, which is a more powerful set. What's more, for both sets I ordered release booster boxes. Magic is not an investment, its OK if I buy a bunch of jank, it just better be very entertaining jank, and it is.
2. I want and need Wizards to slowly unveil the product in the way only they know how to do it. I don't want a random someone's dumpster diving experience. I want and need the creators of the product to reveal it to me according to THEIR plan, not anyone else's. How would you like the preview to a movie you really wanted to see to come from boot leg video from the set? What if the bootleg video is of a scene that didn't even make the final cut? What if the movie had a sequel and you could get the script for it? Would you like to know how the second movie ends before the second one comes out? Well, that's what an unauthorized leak can do: ruin the fun of consuming the movie scene by scene, and then doing the same with the next movie in the series. What a way to ruin all that fun!
That's it. I am sure I could come up with more points, but these two are it: its entertainment, let's leave it to the professionals.
Of course, stealing intellectual property sucks too, but my perspective is more that of a player than that of an intellectual property advocate.
February 6, 2016 update: looks like Shadows Over Innistrad is also the victim of unauthorized leaks. This is lame. I was looking forward to a slow unveil by the mothership.