I once pair-programmed an IK animation harness. The math was hard (for me anyway). There were plenty of times where I'd have balked and gone and made a cup of coffee but my partner had the insight and we plowed ahead. Sometimes I was the one with the clarity. The increase in productivity is vastly more than just twice as fast. Sometimes it can mean that a feature that could not be done can now be done. Or take days instead of weeks.

For the other 80% of what goes into any software: total waste of time, or worse. Two people, independently, could get twice as much done. But worse: pairing on something like that can result in over engineering, language battles, just about any kind of drama to make the day more interesting. Seen it happen.

If I had to choose between 100% pair-programming or 100% single-programming, I'd have to choose pair-programming. In this artificial comparison, pair-programming is more productive. Which is why, I think, so many companies fall into this trap. 100% PP teams beat 100% lone-coder teams.

However, teams that pair when appropriate will destroy the zealots of either denomination. Rationality vs religion.