Some monsters aren't really monsters. They're really obstacles.
What's an obstacle? Something that is in your way. It isn't inherently dangerous or harmful, it's an impediment to easy movement. It's something you need to overcome or bypass to continue on your way. A door is an obstacle. A crevasse is an obstacle. A wall is an obstacle. They act to slow down or channel progress. They don't harm you but they take up time, resources, and attention - all of which make you vulnerable to real monsters. The fortified entrance - and its pit - of my megadungeon is like that. It's a pain to cross into and out of the dungeon, so the PCs have to take that into account when making trips. It limits options.
Take a 10' pit, a "standard" dungeon obstacle. It's not really dangerous. In GURPS DF it's a 1d+2 crushing damage fall - 3-7, average 5.5 - which is nothing to most delvers unless they're lightly armored. Even then, it's probably not any real damage.
A pit isn't really a trap. It's easy to avoid, low damage, and easy to bypass.
If you have to get by that pit in a bad circumstances, well, it can potentially hurt. It's not easy to avoid if you're running in the dark. The low damage is a problem if you're already wounded. It's a real obstacle if you're dragging your friend's unconscious body or his corpse or his petrified remains out of the dungeon. It's a problem if you are being pursued and suddenly the time to cross it is critical.
They can also be dangerous in conjunction with an actually lethal monster. Pits backed by dangerous ranged attackers mean your melee guys can't run up and engage them. A locked heavy door backed by a monster that can attack without line of sight (say, a tentacle-headed humanoid psi type) is now a real problem door. So is one behind you, if you need to run. Alone, it's harmless to the alert or the ready but now it's not your only problem.
Monsters can be like that too - basically, an obstacle, not an enemy. Green slime in D&D, say - not a threat to the alert. Piercers are harmless to people looking forward and up, and trappers to those who don't blinding run into rooms to grab obvious bait chests. GURPS Dungeon Fantasy 2 calls out some of these monsters as non-monsters. Nasty fungi, oozes, and other non-moving critters are merely "Nasty Surprises" (DF 2, p. 18) rather than monsters. There are a few in DFM1, too - the horrid skull and leaping leeches, for example. Not a real threat by themselves, or in isolation, but they can be dangerous to the weak, unprepared, and unwary. They also consume time and resources.
The gelatinous cube-clone I posted yesterday is such an obstacle - such a "nasty surprise." It's slower than an armored man. It's harmless against any real armor. It can't hurt the wary, and you can just walk away from it. It's a Move 2 critter that can't hurt you unless you get completely surrounded by it. It's not hard to kill or hard to clear. Unlewss it's in a 10' pit and you fall it, you're probably going to just destroy it and move on. It's not really a monster. Basically, it's a way to:
- explain why corpses, etc. disappear.
- why chalk marks and such disappear from walls when your players ask why you didn't mention their chalk marks from eight sessions ago.
- make it dangerous to leave wounded companions behind, or leave corpses behind for later recovery and resurrection.
- it's a way to making it dangerous to let your guard down.
- it's a mobile obstacle, so previously "safe" places might not be safe this time. Or later on.
- it makes it dangerous to run in a dungeon, even to try and run out of the dungeon.
But it's not a foe any more than that 10' pit is.
Lots of monsters work that way, in GURPS and in dungeon gaming in general. Most of the time, they're irritating at most and generally harmless. They consume some time and possibly resources (my players needed to expend flammables to clear out the cube to avoid walking in it.) They make it harder to get around when wounded, tired, burdened by loot and wounded friendlies, and otherwise make it tricky to navigate the dungeon. That's the criteria I use to build those kind of monsters.
Now, of course, some obstacles can be lethal - hidden, trapped, and otherwise made extremely dangerous. But they don't start that way automatically. Monsters don't all have to be lethal to be worth notice, either, or useful for the GM or for generating some fun play.
So not all monsters need to be a head-to-head threat against an armed, alert delver. It's enough if they make it unsafe to be other than an armed, alert delver . . .
(Oh, and they are murder at the bottom of a pit, if you fall in.)