For a 200' dive with a bottom time of 25 minutes (fairly normal runtime in 40 degree water) I save maybe five minutes by carrying a second bottle with 100%. I spend that five minutes in putting it on, whacking it on every bulkhead door, switching gases, and then clipping it off.
Below 200' I add another bottle.
The low PO2 isn't my concern, I have never been deep enough to worry about a 12% bottom mix. I just hate the whippet sensation going from 55% helium to no helium to 55% helium to no helium. I prefer to switch off helium and remain off it unless there is a reason to go back on it.