I think the idea is that by writing less buggy code with two people you save in refactoring, bug squashing, and fire management down the road.

Similar to testing I suppose.

No idea how the math works out in the end, I assume it'd be highly variable based on the product being built and the people involved.

What little pair programming experience I have makes me think it’s great to do now and then, but maybe not all day every day. You get to know your team members, share skills, see different perspectives and ways of approaching a problem, and understand how your way of working will affect others ability to do their work. Do it once a week or so and I’m confident it will be good for the quality of the code and the team.