D&D spells can be divided into three camps. There are the Favourites, which everybody recognises to be not just useful but potentially game-changing in the sense that once a magic-user has access to them, they act as force multipliers or otherwise radically shift what the party is capable of (Fireball, Sleep, Web, Charm Person). Then there are the Utilities, which are helpful in solving commonly encountered problems or provide a bit of a boost in combat but which don't in themselves have the capactiy to alter the trajectory of an adventure or dungeon delve (Protection from Evil, Detect Magic, ESP, Mirror Image, Hold Person, Light). And then there are the Overlooked, which comprises that grab bag of spells which often sound charming and intriguing on paper, but which in practice - in my experience - are rarely if ever deployed (Feather Fall, Message, Deeppockets, Taunt, Wizard Mark, Fool's Gold, Magic Mouth).
The existence of Overlooked spells suggests a large gap between the game as it exists in the imagination of the designers and what it looks like in reality. D&D's design, in other words, contains a lot of redundancy. It creates the impression of being much bigger than what it tends to be in practice.
Let's examine one of the spells I just labelled Overlooked - Magic Mouth. Here is the substantive description (taken from the 2nd edition AD&D PHB):
When this spell is cast, the wizard imbues the chosen object with an enchanted mouth that suddenly appears and speaks its message when a specified event occurs. The message, which must be of 25 words or less, can be in any language known by the spellcaster, and can be delivered over a period of one turn. The mouth cannot speak magical spells or use command words. It does, however, move to the words articulated -- if it is placed upon a statue, the mouth of the statue would actually move and appear to speak. Of course, the magic mouth can be placed upon a tree, rock, door, or any other object, excluding intelligent members of the animal or vegetable kingdoms.
The spell functions when specific conditions are fulfilled, according to the command of the spellcaster. Some examples are to speak "to the first creature that touches you," or "to the first creature that passes within 30 feet." Commands can be as general or as detailed as desired, although only visual and audible triggers can be used, such as the following: "Speak only when a venerable female human carrying a sack of groat clusters sits crosslegged within 1 foot." Such visual triggers can react to a character using the disguise ability....
This is a nice idea. Reading it, one instantly conjures in one's mind's eye a hundred different scenarios in which this kind of thing might be useful in a high fantasy novel or even in a notional D&D campaign in which for some reason the PCs want to trick an opponent or pass on a secret message. And I think it's fair to say that the concept is also intrinsically evocative and 'magical' in the prosaic sense. I love the idea of playing in a D&D campaign in which tricksiness, artfulness, conspiracy and bluff are the main focus and a spell like this would come into its own.
I love the idea of it and so I think did the people who made up the spell. But the trouble is that in 'actually existing D&D' the incentives for the players point much more towards memorising and deploying spells for directly instrumental purposes - hurting opponents, protecting themelves, navigating/finding things, overcoming obstacles, and so on. While in theory Magic Mouth will have its uses and also possesses plenty of charm, in reality it will be, well, overlooked.*
The ambitions of D&D have, then, always been much broader than what its rules tend to facilitate. Its goal has, over time, been to encompass all of the possibilities in fantasy fiction within its purview, despite its core rules fundamentally being geared towards a much narrower style of play. I don't blame it for this - and in fact I think this maximalist approach has its virtues (I much prefer D&D's rambling 'incoherence' to the much tighter and less loveable modern games). But it is notable just how much is hinted at within the pages of its rulebooks that almost never finds its way to actual gaming tables. There are many paths which are suggested at but not taken - paths which lead to very different campaigning possibilities, where Magic Mouth would be an important piece of a magic-user's armoury, and an altogether more fantastical and thoughtful style might develop.