Oh, it's a marketing ploy, all right.
Let's face it...a new expansion comes out (2nd or 3rd set in a block), and you go buy one booster box.
A collector...
...trades his dupes away for the rares he didn't get...trades away a couple of extra cards for the rest of the rares. Sees that he has every uncommon and common, shrugs, and stops buying the set.
A player...
...trades away the rares he doesn't want for rares he does, trades away the uncommons he doesn't want for the uncommons he does, and binders the remainder for trade for future sets. Maybe doesn't even buy a box of one of the two sets.
So after one booster box, both kinds of buyers are satiated.
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Now, let's look at the new plan.
A collector...
...looks at his rares and realizes that even if he trades every dupe away for a card he doesn't have, he'd still only have about 2/3rds of the set. This means he'll either buy another booster box, or trade spares away...and I do mean a lot of spares...for the rares he needs. Will *probably* have every uncommon he needs and every common, but still.
A player...
...has decreased odds of getting the rares and uncommons out of each set that he wants, so he'll have to trade for them. Of course, with larger sets, there will be more in each set that he wants.
This is definitely a marketing ploy to increase demand for packs of the second and third set of each block, and to make each rare a little harder to get - as it is, if you buy a box of an expansion set, you've got a VERY good chance of getting any given rare.
Not that I mind.