I'm certainly no opponent of pair programming, but its proponents often seem not to address the 0th order argument against it: the parallel/series problem. To complete tasks <i>A</i> and <i>B</i>, Alice and Bob can finish them in parallel if they work alone, but can finish them only in series if they work together. This means that, for equal-sized tasks, solo programming is <i>a priori</i> twice as productive as pair programming. I'm perfectly willing to believe that the benefits of pairing can overcome this factor (and more), but that's a strong claim and requires strong evidence.