What is the most useless spell in the wizarding world?
Apparently, most non-conjuring aspects of Transfiguration!
We thought it was so important to be able to turn something into a more desirable object or animal. Then, in the 4th book, we find Mr. Ollivander getting everything he wants out of nothing, no base object to transform, and doing all this with other wizards' wands:
-—-Mr. Ollivander by atalienart---
Mr. Ollivander ran his fingers along the wand, apparently checking for scratches or bumps; then he muttered, “Orchideous!” and a bunch of flowers burst from the wand-tip. “Very well, very well, it’s in fine working order,” said Mr. Ollivander, scooping up the flowers and handing them to Fleur with her wand. “Mr. Diggory, you next.”.. “Yes . . . hornbeam and dragon heartstring?” he shot at Krum, who nodded. “Rather thicker than one usually sees . . . quite rigid . . . ten and a quarter inches . . . Avis!” The hornbeam wand let off a blast like a gun, and a number of small, twittering birds flew out of the end and through the open window into the watery sunlight… Mr. Ollivander spent much longer examining Harry’s wand than anyone else’s. Eventually, however, he made a fountain of wine shoot out of it, and handed it back to Harry, announcing that it was still in perfect condition. -JK Rowling, Harry Potter and the Goblet of Fire (Kindle, ed.)
Yep! You read that correctly. Mr. Ollivander, not even from his own personal wand, produced flowers (living vegetation), birds (living animals), and wine (a palatable beverage) all from the wand itself, without performing any transformations that we know of, conjuring them from thin air. It is purely Conjuring and Charms!
Indeed, we find Professor Flitwick teaching his class the wine-charm in the 6th book. And Hermione, quite famously, uses Avis to attack Ron in her jealousy (technically taught in Transfiguration, as Joyce Yang notes in comments, but not really transfiguring one thing into another):
—— Bird-Conjuring Charm ——
Here, Hermione uses a very similar conjuring or charm to create, out of nothing, a flowery wreath at the graves of Harry’s parents:
So, Ms. Augusta Longbottom, is Charms really “a soft option”? No! It may be the only needed option and one can just take-up Transfiguration as a very secondary and just-for-fun elective (like Divination!).