As a new player I really don't see the harm in this being added at all. Though it would make perfect sense to have it as an init setting for those that don't want it or don't want to have to worry with designing fully working sewage system. It would be nice if the whole "producing waste" task happened only when dwarfs are on break. Somehow I could almost forgive my pump operator (saving the fort from excess
magma fun) abandoning his job to attend to his bodily functions rather than just being bored with work.
I won't say too much since what I think has already been said over and over.
Although one of the things I found strange with Dwarf Fortress is that it is very easy to have a self sustainable fortress, the whole producing waste makes a self sustainable fortress just that bit harder to reach which seems fair to me. That said dealing with waste should be simple enough, either dig some holes and pit it with buckets or make a complex sewage system that can attend to the needs of a full population of boozers. I'm a bit iffy about animals creating filth though, they're already annoying enough with their children and their hatred of high frame rates.
2) Potash makers will become more useful. Change their title to Fertilizer maker, add a reaction to combine potash and manure to make fertilizer. You can also add the ability to fertilize crops with potash, manure or fertilizer.
3) I don't advocate giving adventurers the ability to squeeze one out whenever they want. Consider it abstracted into traveling. However, adventurers exploring sewers should have the option of flinging poo.
For 2) that makes sense and could open up the opportunity to have a lower class dwarf that has to live his life digging through sewage to fertilize crops. The benefits are higher crop yield but at the cost of higher rates of disease among these unhappy labourers (might even help to create a crime system where unhappy poor
rat hunters dwarves turn to stealing).
As for 3) This seems to be the main thing people seem to have against the idea of sewage in dwarf fortress. I agree with the whole "only producing waste while travelling since it makes enough sense" I don't think the whole "crap as a weapon" should be added only since it just seems strange to throw sewage at something that lives in sewage. I doubt throwing some sludge at a monster is really gonna stun it enough to consider twice about biting off your face, might even make the thing angrier. Besides the point of the filith is to add an extra layer of thought to designing rather than something that is meant to be "hi-larious".