Space savings too.
I agree there is no weight issue but as with everything else in airplane design, physical space and capacity can become critical issues.
Why take up space in the toilet tank with soapy water and old coffee? Better to jettison that stuff and save the space for the real vile stuff.
ABX--sounds like the cabin crew were just being snooty about using the lavs on the ground and getting in their way. They can be like that.
True, the sink liquids get drained out of a heated mast. On the ground they just drain on to the ramp. In flight, at *very* cold temperatures, most likely, it vaporizes.