A flat spare does leave you pretty screwed. I used to drive a Beetle and that used the air pressure in the spare to drive the windscreen sprays. I assume it might need pumping up to compensate now and then. I bet most people rarely check their spare.
I must confess I have never checked my spare. I kinda assumed that it would be checked during my MOT but I think, like the Aircon fluid, it might be something that they don't do!