Considering I once designed a fort based around having a sewage system/water source (See Wavehandle). I would consider it an intersting addition for challenge-sake. Let's not forget the added realism, and with disease and infections implemented, it can serve as a powerful deterrent and bad mood generator. It can also be taken advantage of as a form of fertilizer and applying the art of Dorodango ala a Mythbusters myth, you can construct blocks made from it and use it as building material (or craft shiny spheres from mud or "mud"), like some of those houses that are sorta adobe-like, but made of a combination of mud and crap. Alternatively, said mounds can also be used for catapulting or making ballista, arrows, and bolts disease ridden and adequate deterrents. Miasma would be less of a worry compared to these acidic cloud generators.
Despite it's crudeness, it's "crude" quality could provide a challenge as well as a share of benefits and consequences, provided it's handled properly. And I would advise against sharing it's disposal with the drinking source. Don't want to get other infections from that, unless disposal is dealt with in the nearby stream/brook, A latrine system would have to be implemented, and I'm sure Toady wouldn't appreciate many of the other reasons behind implementing the code, but at the same time, it would be an interesting programming challenge just for the hell of it.
But yeah, I wouldn't mind it being included; just as long as it would be shut off upon install. It's at the player's discretion to allow defecation.