Originally Posted by
Imbalance
DM caveat is the short answer. As a primarily non-combat spell, it allows the injection of a powerful character for story purposes without the side effects of a full strength dose of DMPC wizard.
I can imagine a host of answers to "why," but I don't think you'll be satisfied by my take. I simply accept that not everything created by the developers is useful or a good idea. I must've played through Earthbound a dozen times, and while I'm sure I always completed the side quest to return the contact lense to the dude in the bakery, I can't remember ever using the reward. It was similar with the Biggoron Sword, and countless other items/techniques/unlockables - many things exist in games for no other purpose than to be an interesting option, and the only real enticement is that they're locked behind a higher level or specific achievement. I find that to be the case with at least a quarter of the spells in D&D, and, based on my video-game-derived prior experience, those are the ones that perhaps ought to be dangled in front of the players like a carrot to chase and acquire, then decide the worthiness for themselves. The difference is that with tabletop, the freedom exists to "alter the code" each playthrough, so that if you and the DM can imagine it, you may choose to either improve and strengthen the spell or press up, up, down, down, etc. to access it at an earlier stage.