I'd like to see sewers long, long before we have anything like guns or explosives. I, for one, actually enjoy the pre-Renaissance feel that their absence brings. Thankfully, we can have sewers without gunpowder.
This is a simulation of the hardships of prying civilization from the clutches of the wilderness. As such, I think it'd be strange if DF doesn't have waste management infrastructure, as part of the simulation.
Sure we already have refuse haulers, but healthy sanitary systems are one of the things that's really necessary for a city to develop. Even primitive solutions like throwing it in the rivers, or guttering it to the outskirts of town are better than nothing.
We already deal with the other messy realities of survival, in a reasonably mature fashion. Our dwarves are able to defend themselves, kill their food, and clean vomit off their floors without the game becoming unnecessarily disgusting, and I think this possible for sewers, too.
Some players are going to dump goblins into cesspits, yes. However, if this stuff relates back to healthcare and it's proximity create miasma and disease, most people are going to be more concerned with moving it away from their dwarves, than under their goblins. The health care changes that are going in are almost certainly necessary groundwork for that, but they're going in anyway, right?
Manure being a cheap (but somewhat dangerous, considering the increased risk of plague) source of fertilizer on low wood maps would also increase the number of places that dwarves could survive, once farming gets nerfed back.
I don't know that dwarves should be willing to shovel un-processed dwarven sewage onto their fields though. It's not a good idea, and dwarves do seem kind of tidy.
Manure and sewage might should be kept distinct, though I don't see any purpose behind separating liquid and solid waste. It's not that hard to keep them separate enough that they could have different uses, as dwarves shouldn't poop on the floor and animals won't use the latrine .
If there is no latrine a dwarf should just find a patch of dirt and bury his business, like people do when they are working or camping in the wilderness. It shouldn't make dwarves happy if they have to keep doing it for years, and might should debuff their disease resistance (or whatever the penalty for doing something unsanitary is) so as to encourage players to provide proper facilities suited to the size of their fortress.
You can't keep doing that indefinitely, with a very large group, as you tend to end up digging up as much as you bury, but unhappiness and poor health are probably as deep as we want to delve into that. Trust me; I've worked wildfires where large teams were cut off from port-a-potty pumpers, for extended periods. It's fundamentally different from a group of around seven people camping in the same place without a proper latrine.
Dumping manure into sewage should just turn it all into sewage.
I do agree that sewage should produce miasma as it evaporates. Open air sewage treatment vats are not a thing anyone wants to be close to.