I know I sold it. But those of you who hang on to everything you find in the game have a reason to celebrate: the Luffa is back. Apparently in Karazhan, there's a rogue boss named Moroes who lays down a debuff called "Garrote"-- it's a whopping 1000 damage per 3 seconds for five minutes (or until he dies, as of 2.0.6), which is a lot of damage. And yet, while Stoneform, Divine Shield and Ice Block will all remove it, the only other way you'll get it off is, you guessed it, the ol' Luffa.
If, like me, you did vendor it, don't worry too much-- the Luffa only works sometimes, and the fight's not impossible without it anyway. But a few players are feeling a little regret at tossing away something they'd never thought they'd need. Maybe a future quest in Outland will someday grant us an extra absorbent Luffa for future use.
Update: And here's even more reason to keep it. Commenter Wari asks about Rokmar the Crackler's debuff in the Slave Pens (he drops a bleed that does damage until the player who has it is healed to full), and it looks like the Luffa will clean that annoying buff off too. Who knew the Luffa would be so useful?