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.