If the barrier is simply a pay wall to ensure that those who pirated the game(i.e. didn't pay for it) don't have access to a substantial part of the game, then why charge $10? Seem like just $1 or fifty cents would do the trick just fine(remember piracy only has three reasons: lazy, cheap, and a douchebag). Also, what's to stop the pirate from just paying the $10 for the code and getting full access to the game? He still comes out far ahead of anyone who paid full price or even bought the game used, because he only paid $10 total while everyone else is paying $60-$70 total(Gamestop is not much of a discount considering they would sell the game for no more than $5 off the new price).
The other question is whether the multiplayer is really that substantial or valued-added compared to the single-player. From what I've been hearing of most games that try this, the multiplayer is a bolted-on after-thought, adding little to the experience of the game. In some cases, it's been deleterious to the overall game experience. So, are the pirates really losing out by not paying to get multiplayer and just sticking with enjoying the single-player? What is there compelling to the multi-player that makes it the better experience such it is the real reason to buy the game, making the pay-wall actual be effectual, rather than it being overshadowed by the single-player experience or passed over entirely by the vast majority of gamers? If there is no true reason to buy the multi-player, then this whole scheme will just be a bogus money grab that will only serve to cost more money in managing it than is made from it(or at least it would, if gamers weren't such un-self-disciplined addicts who are easily suckered out of their money cause they can't go two shits without a game to play).
What if gamers simply decide they've had enough and just stop buying games from these companies altogether(given history, there seems little chance of this, and the publishers know it)? Gamers just decide to find other interests or focus on games that don't pull this BS. These companies would fold. They need and want your money more than you do. So, I think the reality is that gamers, as paying customers, actually have the real power; they just fail to exercise it properly, preferring, instead, to merely complain, bitterly, while still giving away their money to the same companies hurting them. You don't want, don't buy it. Don't pirate it, either, but just don't buy it.