Having been on both the knowledgeable and noob side of this "pair programming onboarding", I find it to be fantastic. When furiously googling / grepping around the code to self-onboard, it is so difficult to see the bigger picture of how the system works; if someone can explain the architecture to you, you become much more self sufficient, very quickly too.

On the "mentor" side of such pair programming, it is extremely useful to have to verbalize things that you may have gotten away with only fuzzy knowledge of previously. When the new hire asks a question that makes you say "oh wow that actually is quite broken!", you win as well!