I'm genuinely curious about the meaning of your last line. Do you mean missing out on the quality of having two people looking at and collaborating on the same code (vs solo), or the quality of sharing the knowledge with the junior developer long term? or both?
The former. Pairing feels subjectively like a great way to teach, but I'm not sure how much difference it actually makes - those who want to learn will find a way, and those who don't always manage to avoid it somehow.