But here's the math:
A small set has 55 rare, uncommons, and commons. Let's say you want a player's set of all the uncommons and commons. So, with perfectly even distribution, you need to open enough packs to see 220 uncommons. At 3 uncommons per pack that's 73.3 packs - or 74 packs. At 36 packs per box that's 2 whole boxes plus 2 more packs on the side. This will cost most people between 120 and 160 dollars, and even then you're not likely to get your player's set.
But let's assume the distribution is perfect and 74 packs get you your full player's set of non-rares. If you can easily pre-order a full set of this stuff for 40 bucks, this means your paying 80-120 dollars for 74 rares, which might not sound so bad, but again, if the distribution is even this only comes out to 1.345 copies of each rare, which isn't too useful. With rares you'd rather have 4 copies of the ones you like and ignore the rest. I'd rather spend that 80-120 dollars buying 4x sets of the rares I actually want to play than be stuck with 1 or maybe 2 of every rare. It's not very fun being stuck with only one copy of any card.
"But I can trade" you say. MAybe, but with most people wanting roughly the same rares, trading with a hodgepodge collection of new rares usually doesn't get you as far as you would like it to.
Oh, and of course tihngs get worse when you're talking about a large set. Buying full sets of commons and uncommons is the way to go for me. I spend way less money on the game now than I did.
I know not everybody needs to "keep up" with all the sets, but at the same time we're hearing stories of somebody buying a whole box just so they could get a couple copies of an uncommon and a single copy of a rare when there's really a much cheaper way to go about doing that.