That's a disgusting idea! We use the little shampoo bottles at the hotel, and one of my biggest pet peeves is the stupid body wash container affixed to the wall with years of grubby handprints on it, so we bring our own soap bar. The latest outrage was a 5-star hotel that inflicted that awful recycled wafer-thin cardboard non-absorbent toilet paper on us - we won't stay there again.
Any hotel that tried to give us used soap would be off our list!