Wandering monsters are an important component of the megadungeon experience. From a story perspective, they create the illusion that the dungeon is an inhabited place with an ecosystem of monsters that have their own agendas and missions - they don't just lurk in the dark, waiting for adventurers. Conceivably there are tombs and smaller non-living dungeons where the monsters do just lurk and wait, lurk and wait, but the megadungeon is giant, and has a panoply of critters. Wandering monsters set the thing in motion and ensure the dungeon remains a hostile environment.
From a game perspective, wandering monsters exert positive pressure on the action. They ensure players don't endlessly search without consequence; they drain party resources; they are a principal tool you have as a referee to create problems for the "15 minute work day". The 15 minute work day refers to entering the dungeon, visiting a room, expending all the party's resources at once, immediately leaving, resting overnight, returning to the dungeon, and doing it all over again… Your players might still have to abridge their delve for legitimate reasons (such as having their keisters handed to them) but wandering monsters ensure it's not a great strategy. Resource management and planning is one of the skill aspects tested during extended dungeon exploration.
In very large dungeons, the irritation factor of wandering monsters also increases the worth of elevators, secret stairs and short cuts, and alternate dungeon entrances. Anything that cuts out superfluous encounters has value.
However, with my current campaign, I am noticing a few things I don't like due to our constraints. First, I'm trying to run shorter pickup games here and there, sometimes only 2 hours or less. Hitting one or two wandering monsters while trying to reach the unexplored areas can lop off a fair percentage of table time when the sessions are so short. Similarly, if we're targeting a specific end time (like 8:30 for a school night), the players want to keep exploring right up until 8:29 and 59 seconds… not leaving any time in case they hit wandering monsters on the way out of the dungeon. (I could certainly make them stop 15 minutes early each night to account for travel, but that's not entirely satisfying, especially if I don't roll encounters and they could have used the time).
With these concerns now expressed, I'm considering some adjustments to the standard approach for our short games:
- When traversing areas that have been cleared by adventurers (and are slowly re-stocking), the incidents of wandering monsters is greatly reduced - 1 in 6 per hour instead of 1 in 6 three times per hour, for instance.
- When leaving the dungeon at the very end of a session (to meet our time constraints), the wandering monster rules are abridged.
- When leaving the dungeon at other times, roll checks as normal.
However, I'm not suggesting to ignore the dice if I don't like a wandering monster result… dice fudging is verboten. If it's worth using the dice to determine an outcome, it's worth rolling them in the open and letting the dice fall. Dice create drama and consequences, and using them temporarily shifts the referee into a spectator of the unfolding story much like the players - none of us know what's going to happen when the dice are rolling, and that's exciting. Dice are the neutral arbiters of fate. If you don't want to the possibility of a negative result, don't even roll them. Make a ruling instead.