Actually release is important for another reason and that is getting popularity and reviews. Companies don't actually make the significant amount of sales from launch day, these are usually as you said impulse buyers and early adopters. If you look at CoD MW2 it only had 4.7 million sold on launch day and another 4 million in the subsequent 5 days a fraction of the 25 million that bought the game later by March 2010. People don't usually buy the game as soon as it comes out, the majority of the market wait it out until they are confident in purchasing the product.
Battlefield will sell more units after the first week once the rest of the market is assured its a quality game or are pressured into buying it from hype or their friends. Marcipaans is right in saying that piracy doesn't effect games as everyone thinks it does. The people who pirate never intended to buy the game in the first place or are testing the game to see if its worth buying.





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