Will there be a need for toilets sometime?
It's been the subject of some of the most heated arguments in the Suggestions Forum, since it's one of the few things suggested that can get people really up in arms (besides homosexuality). Specifically, there's the fear that it will result in something like people making "poop moats" that they drown elves in or something, and DF having its reputation warped into "that game where people drown elves in poop". (Of course, the already-extant atrocities we commit daily and the fact that there are things like mass vomitings in this game don't seem to be considered in this context...)
Although it's mostly shaded by my heavy involvement with the Farming threads, the biggest argument for biological waste products I value, however, is that it links up the Nitrogen Cycle, and serves as a critical means of recycling other nutrients in farmlands.
Or, more bluntly, poop, even dwarf poop, is valuable manure for farming.
In a world before the Haber-Bosch process invented the capacity to artificially create nitrogen or other fertilizers, you basically couldn't just take nutrients out of the soil and just replace the nutrients with something you buy in a bag from the store. What you took from the soil, you had to return.
In fact, even in the modern day, 70% of all the nitrogen in fertilizer is nothing more than decomposed urine. (And urine is the body's way of getting rid of excess nitrogen from protein you've consumed.) It's just more efficient that way.
Hence, yes, I like sewers, too, but more than that, it lets you change the game from a place where food is spawned ex-nihilo from wet dirt into an actual ecosystem simulator with conservation of mass. For dwarves to live, they must eat the crops, for crops to live, they must eat the soil, and to replenish the soil, you must decompose the waste. (Plus, if we can decompose goblin and animal corpses as well, you create a serious "ashes to ashes" theme...)