I regard crawling into a devil's mouth as skillful play - something interesting and exciting is more likely to happen and that's what I want when I play an rpg. Prodding everything with a 10' pole, or getting your orc slaves to enter every room first, is the play style Tomb of Horrors encourages. This, to me, is a bad play style.
Judging the
Tomb of Horrors as if it were supposed to fit into a normal campaign is to take the module out of context. It's a special, one-shot dungeon that's specifically designed to do something that
isn't the normal style of play.
And when it's used in its proper context, it's a fun little scenario.
EDIT: Using the 10' pole on the devil's mouth won't even tell you it's dangerous. It comes back with the end missing, but it's reasonable to assume it's been teleported rather than disintegrated. The former being more common dungeon weirdness than the latter. I think this is what people mean by arbitrary - it could be a teleporter, it could be a disintegrator, how are you to know?
I've run the module several times. Every group that has used the 10' pole trick has correctly intuited that there's something wrong with the demon's mouth. (Largely because there's an
actual) magic portal right next to it that they can compare behaviors with: The archway gives them their pole back; the demon's mouth doesn't. Even when they've considered the possibility that the end of the pole has simply been teleported away, they hedge their bets and go for the known-safe option.)
Other successful mouth-avoidance techniques: Correctly interpreting the poem ("shun green if you can"; the demon bas relief is green). Throwing an object through both portals and then using a
locate object spell. Divination spells. And an
identify spell.
In fact, I've only had three fatality incidents involving the demon's mouth: One group sent two PCs through before bothering to do any kind of investigation. Another group thought of the
locate object tactic but didn't have the right spell prepared, so they decided to use
locate person instead to figure out where the portal went (only to discover it wasn't a portal). And a third group dead-ended elsewhere in the tomb and back-tracked to the demon's mouth to "see where the other portal went" (having concluded that it must
not be dangerous like they thought, since there wasn't anywhere else for them to go -- they were wrong on both counts).
In other words, every single group I've ever played with has correctly determined that the demon mouth was dangerous once they decided to actually investigate it. (Although in one case they later out-thought themselves back into the wrong conclusion.)
I will also give a dishonorable mention to the guy who saw what happened to the pole and then stuck his hand in to "see if it will do that with organic stuff, too". (It did.)