I find it best to NOT pair program by default, ever. To date I haven't met a good developer that doesn't degrade in performance when being watched.

For Junior developers if helps, mainly because the most experienced developer can explain the coding mindset and codebase gotchas.

Having said that for rapid prototyping things that require a lot of knowledge in different domains, getting two SMEs in one room writing code without interruption can work well...for me. You can't force it, you can't mandate it, you can't expect it. Developers would do it themselves.

In my case, if I want to work on something complex and new, and I know part of the domain but not all of it, having someone that knows the other side can make things go much much faster. Replicating or adding little features I need to be left alone and just do the work.

I might agree that in the beginning some developers aren't as efficient as being solo. But I've yet to find someone who doesn't benefit from being forced to explain their thought process in a clear and concise manner to a coworker. It benefits the pair and it benefits "normal" communication because of that constant practice.

I've found that even for devs who are completely against, which I was one, can come around on pairing given the space and time to succeed.