I definitely agree that it's a skill to be learnt and it's something I enjoy.

Is it really useful to do it all the time though? Like there must sometimes be edge cases where people will achieve more alone, e.g. if it's something simple/repetitive, isn't it more productive to just hammer through?

Sometimes, yes, it would be as effective to solo. But we're reluctant to break pairs because it means the same fan-in/fan-out problem reappears.

So for repetitive stuff we often wind up throwing together something to do it for us.

Additionally, in Pivotal Labs, one goal is to teach the practice of pairing. It works best by total immersion.

By analogy, French teachers try to make the class speak French at all times, even if sometimes it's easier to speak a mix of English and French.