Lets say, hypothetically, that you can use it to ramp any programmer up to being an expert on the system. Maybe that takes two months? Or even six months? Once you have a team full of experts, you're now spending two programmer hours to do every bit of work that could have been done in one programmer hour by a single expert.
Unless you have such incredibly high turnover that the period to transform a developer into an expert is close to or less than the average tenure of an employee, in which case your team has other issues that likely need to be addressed, because why are people jumping ship so quickly if it's such a great place to work? (For example: If it takes six months to produce an expert, you'd need to have an average tenure of less than or equal to six months to really benefit from pairing; anything more and the 50% performance penalty has to eat into your overall productivity.)
On top of that, I have never been at a job where it's taken me more than a week to get up to speed on a system, or at least to the point where I can simply ask the occasional question of the experts on a team to get me unstuck. Even if the average developer took a month or two, pairing beyond that point is simply fun and not more productive than two programmers developing independently.
Unless one of the developers is otherwise likely to produce negative productivity on their own, I suppose. I've worked with that kind of developer as well, and the right answer is to eject them from your team, not pay other developers to babysit them.