Admittedly we won't see this in future due to Toady's improvements to dwarven work effort in the next version but:
Urist cancels job: Gone to the bathroom
Really this stuff may mess around with the rating (honestly no clue how, but then violence potential probably outranks it already)
If it is put in the frequency with which nature calls should be something similar to once a season or something. Though interestingly I'd say that your sewage system should actually fill with anything upon use by a dwarf instead, your population should determine how much volume there is too handle (probably refilled at the start of every season, or constantly depending how you handle it), naturally you should find use for it somehow, fertilising fields via clever plumbing (try not to open the wrong floodgate), dumping it on your enemies for defense puproses (do dwarves outlaw biological warfare?), drowning nobles/elves, creating sewers would be cool though.
Only matter is that plumbing technology is a very debatable matter for dwarves if you ask me, the romans had a sort of plumbing system (even aztecs or mayans had running water plumbing) so I'm not disputing sewers, but these are only effective if you don't let them stagnate (such liquids stagnating ought to be devestating if you approach one of its entry points), so dumping it into rivers, bottomless pits, magma pipes (yellow obsidian anyone?) each with their own consequences given the upcoming versions potentially aggressive nature. Though if we say their plumbing would be more medieval... thoughts of miasma pits, filthy streets and such come to mind which don't seem so fun, but is the age and technology with which the dwarves are supposed to embody more. Then again they get to use magma furnaces.
Also a thought on the dumping matter to me would be, water flow throughout the system from a source to the final destination where the waste just falls in and may be detoured once or twice before being dumped (say over a fallow field or goblin siege) then reaches its final destination of doom, a property given to it though, no fortifications allowed, it'd prevent stuff from leaving and cause it to stagnate (though adding a potential filter property to fortifications of varying kinds is also interesting, maybe even methods of purification)
I'd say this abstracts from the issue no need to mention the matter and just denotes fluids and such involved to 'sewage water', 'stagnated sewage water' and 'festering sewage water', possibly adding a race tag to it, possibly meaning festering dwarven sewage water can potentially make humans drunk. You could turn the system off like you would turn economy off, its an interesting challenge, could even be good fun. I'm sort of in favour though I can understand it not being in sort of