In the real world, when a person becomes the victim of poison, the effects and symptoms continue for hours, some will even persist for days before killing or subsiding. The rules provided in the Player's Handbook don't really reflect this, mainly because it's just more convenient for DM's to get the poison damage over and done with to reduce the bookkeeping. A GM should do what it takes to make his own life easy, but poison is not a corner to be cut! The role-playing opportunities of agonizing symptoms and uncertainty make poison more realistic, and really is an essential (and often neglected) story-telling tool. If dramatic and imperiled gaming is your goal, it's worth it.