It is always difficult to explain this part to people. That simple typo that stops you for five minutes while typing? Your pair caught it before it ever happened. Saved five minutes right? Not exactly. The stoppage of flow to fix the trivial problem takes your mind off track. It is going to cost you far more than five minutes while you get your mind locked onto finishing your work.
Then there is the removal of code review meetings/gate keeping. Little to no team silos and higher team redundancy. Faster team member ramp up. Lastly, higher code quality.
A simple typo stops me for 2 seconds, because my IDE caught the error and highlighted it. My mind isn't taken off track because by the time I actually get to the typing part, the entire model is already clear in my mind.
"Then there is the removal of code review meetings/gate keeping."
I'm not so sure that pair-written code should be exempt from code reviews. The two people who wrote the code made a bunch of decisions in the heat of the moment (and possibly under time pressure), and both are emotionally invested in the code that they wrote. If it's reviewed later by people who weren't involved in writing the code, it's likely that they'll notice problems that the original programmers did not, or see better ways of doing things.