[henkster]

I read somewhere that the O2 environment allowed them to have a lower air pressure in the cabin, something like 5 to 8 psi, which is a weight saving for the duration of the flight.

[Calvin Locke]

I don't know about saving weight, but the 5 psi atmosphere has other advantages, they trumpeted that up to a 1/2" hole would still allow approximately 10 minutes of atmosphere so that the crew could still be able to don pressure suits, any EVAs means less atmosphere to lose in less time to equalize pressure, and the reverse on after the ingress, etc.

The Soviets were still near 15psi, even after the problems they had with their first EVA. The cosmonaut's suit expanded enough to prohibit him getting back into the capsule, and he had to severely deflate it.

I don't think they went to the advantages of the lower psi until détente, and the joint US-Soviet mission.

There are probably physiological advantages to a lower 100% atmosphere too.