I guess I'm being super naïve here, but could someone please explain why leaked cards are such a big deal?
It's not so clear how much it matters, depending on how far in advance it is. The smart move is probably to try to keep the sets a secret though. Wizards of the Coast's stance is, that they will drum up interest in a Magic set better themselves, via previews, than via uncontrolled leaks. And they believe this so much that they have sued people over it. I feel like, they have done this math; probably the leaks are actually bad.
It seems better to have information about a set show up right around when people can buy it. They read the spoilers, are excited over the awesome new stuff, and then there's the set, available for purchase. If the leak is way in advance, they aren't maintaining that excitement, and you have no tool for building it back up. A lot of people were buying however much of the Magic set anyway, or were buying that Dominion set anyway, but you know, I can see it having a negative effect. The new set is more exciting if you didn't read the cards 6 months ago, and if you don't buy every Dominion expansion, that could make the difference. Then too, if it's spoiled in advance, people have proxied the cards, simulated the cards, found whatever combos. You don't get to be the person on your block who found whatever obvious trick.
So anyway I personally think a Dominion set would like to be spoiled pretty close to release. If it were say two weeks early, that wouldn't matter much; it would just spoil the previews. The previews seem like a good way to promote the sets though; show up each day and find out about more cool stuff, then talk about it, turn it over in your mind, in manageable chunks. So it's probably better for us if those aren't spoiled either.