[SomeCallMeTim]

"Fewer bugs" needs to produce 2x productivity over the productivity of the two developers independently in order to actually be a benefit. Code review and using typesafe languages can also catch nearly 100% of those bugs as well, without the 50% performance penalty.

"Keeps people on task" seems like management-think adjacent to "butts in chairs in an office" requirements: There's a fear that someone might be "wasting" time by being distracted, when you seriously need to contemplate problems sometimes.

I'd need to see that "actual evidence" that we're allegedly discarding. Two programmers with two computers clearly could produce twice the productivity if they're both strong programmers. When I've been pairing with people I simply do all the work. I might get 1-2 comments per hour about a missing semicolon that I would have discovered the moment I tried to build. The claims of the pairing advocates are pretty hard to believe, and I don't see extraordinary evidence to back their extraordinary claims.

As I intimated above, the only way it would work is if the developers are junior (or mediocre) enough to be prone to contributing 0.5x or less productivity left to their own devices, so that the two developers' inadequacies at programming complement each others' and you get a 1.5x productivity out of them combined or something. And I do suspect that companies use that fact to hire less competent developers and make them at least reasonably productive; Pivotal does 100% pairing, and sells it hard, but note that they're paid per developer hour and so actual productivity per-developer isn't what they're necessarily what they wanted to optimize.

As to your Edit: The exact thing I'm objecting to is 100% pairing, which is practiced at some companies like Pivotal. Or worse, 100% mobbing, which is more than two people at one screen.