Funny. Do they expect that people who break one rule (smoking on the airplane) will abide by the other rule and properly dispose of cigarettes?
Assuming that I was a lawbreaker and had decided to smoke in the toilet of an aircraft, for some reason I would want to find somewhere that I could dispose of the lit cigarette without it possibly causing a fire. So, yes, I would use the ashtray provided.
However, I do think it would just be a better idea not to smoke in the first place.
Mind you, I am an ex-smoker, so I don’t get the temptation to smoke anymore anyway!