Bearcat- You wouldn't say that if you were on a busy aeroplane for 11 hours, and the vast majority, if not all your toilets went out! They use a vacuum system, and I think there are 3 tanks altogether. Even if you are down to 1/3 capacity, I think continuing would be rather more unpleasant all round than worth it.
As for losing one engine out of four so you have the capacity of a trijet...you have a problem with continuing?