It's not a black and white thing or a silver bullet of any kind, the concept of pair programming.
Pair programming is generally extremely efficient for various reasons, but done right it's also very exhausting (in my opinion any way) - meaning a good pair might pair program a few hours a day at most. This should be encouraged, of course, but not mandated and especially not mandated at 100% time because that seems hard to manage (with sustained efficiency and no beer anyway).
So, pair programming is just one way of approaching a problem, and I find it hard to look at it from the question's perspective. It's not like you need to hire twice as many developers for this. It's like wondering if it's worth hiring a message boy/girl to have two employees in the same department talk to each other... when the obvious solution is to have them talk to each other directly without an additional messenger.
