Would you not think it is such an important item (how much useful consciousness do you have left once the masks drop?) that it merits a briefing? In Europe so many low-cost carriers are using secondary airfields with short runways and executing bleeds off take-offs to nurse a few extra pounds of thrust out of the engines, then forgetting to switch it on again, that pressurisation warnings are more common that one would like to think. I know of one incident involvng a well known airline where cabin crew thought some pax were showing the early signs of hypoxia.