Our diving is all on extended vacation trips. We might do a 7-12 day liveaboard combined with 5-7 days at a resort. We logged 60 dives on our last trip. Since we travel a long distance to do this, and spend a lot of $$$ in the process, we want to do 4-5 dives every dive day (this is because we love doing it, not because we feel we have to get our $$$ worth). The issue is not increasing bottom time per dive, since dives all end up being somewhere around 65-75 minutes. It's the residual effect of nitrogen that starts to creep up, and Nitrox gives us a better safety margin and allows us to continue getting dives of that length for the duration of the trip. I personally don't think the "feel-good" factor is that significantly different with air or Nitrox after a week of long dive days.