I recently read multiple sources that indicate human-occupied vehicles (Space Shuttle, ISS, Soyuz, etc) are all pressurized to 1 atmosphere of pressure (approx 100 kPa).
Since I believe that people function just fine to an altitude of 8,000 ft, or about 70 kPa, why aren't vehicles pressurized to less than one atmosphere?
(At 70kPa, I believe the atmosphere would be approximately .15 partial pressure of O2 + .56 partial N2 because 0.70 * 0.21 == 0.15 ).
The ratio of N2 / O2 would be the same, but there'd just be a lot less gas around, which leads to various benefits. There would be less O2/N2 gas to launch into orbit, or existing supplies would last longer.