Gavin was wondering about random encounters on Google+. He was wondering about probabilities and said he had noticed that “wandering monsters virtually never come up.”
Yeah, wandering monsters are rare. But they do happen once or twice a session. The effect they have depends on the setup, however. If your players are pressed for time and after two or three hours they need to leave, and thus the dungeon exploration ends, then additional random encounters don’t do much, I think. They sometimes surprise the referee and add some color, that is all. That’s how I run it. I just roll the dice when I’m bored as a minor tax on players taking too long to make decisions or listening and checking for traps all the time.
If you add a severe penalty, as in rolling on a terrible table of tearful results if the party doesn’t make it out in time, then the exciting bit is rolling for random encounters on the way out and hoping for no delays. That’s how I want to run it, but I never rolled on that ominous table and thus perhaps players don’t actually fear it.
If, on the other hand, players stay for as long as they want but they can’t heal or memorize new spells in the dungeon, then the dynamics might change: they try to maximize their stay, pushing resources to the limit, and now avoiding combat with random encounters is even more important. Perhaps that’s how Gary ran his table?