Time-based is exactly what the original poster meant, so I'm aware of that :] That doesn't answer my semi-questions, however. Experience points and PPs you get when you are actually doing something, and learning from it, for example. But what purpose would survival points have? How do you explain them, what is the need for them, what would you do with them, etc.
Take into consideration that with anything time-based, someone can idle in a corner somewhere safe from harm and gather these up to spend on something more beneficial later. Of course you could then suggest that there be limits, perhaps penalties, etc., but is such a system worth pursuing? Is the work to be done on it worthwhile?