It's true that some pirates would have paid if they hadn't been able to pirate. But that's not the whole story: sometimes getting media for free makes people more inclined to buy it or related media later. I've heard on Hacker News that free music distributions can create an audience for live concerts. Another example is "freemium" software and services. At least one study (that I'm too lazy to look up) found that some people will pirate music to try it and then buy it if they like it. So you can't determine from theory whether piracy leads to less or more revenue; you need real data.
I really like that argument and actually hope it would prove to be true - would completely change a significant part of the industry.
However, it is by definition of US law illegal to pirate copyrighted content, so regardless of whether or not we think it is a benefit to content production and distribution companies for us to pirate their content, it is always going to be illegal.
