I agree with this; I am a supporter of sewage, but only if it has somewhere to go. Better farming is also on my wish list.
When sewage is released, it should be alongside better farming.
It also add challenge to stuff like long sieges, meaning that barricading yourself inside indefinately is no longer really a choice. Without a sewage system in place the dwarves will stink the place up causing disease in your fortress, since they normally would do the deed outside without such a system.
Things would biodegrade much slower in the dark and on stone as opposed to clay or sand. The ability to gather up topsoil from outside would normally be essential to sustaining an initial underground farming effort, though there would be an initial rather low nutriant level, highest for peat, middle for clay and low for sand.
Since you get back in sewage less than the nutriant value of the food fed to the dwarves, the only way to continue to grow underground plants would be constantly dig up new soil squares. Yields from substrate would always be considerably lower than those with topsoil, meaning that it would not be long (well several) before a large population starves to death if all they can do is dig up more substrate squares to farm.
An external supply of nutrients, for instance fish from a river would generally be neccesery for the long-term sustainability of any underground farming system. Topsoil on the surface however is constantly being replenished by creatures pooping, rotting away and a system of nitragen fixing based upon certain plants. Rainwater can wash out nutriants and some of that rainwater drips down into the underground soil tiles, slowly adding nutriants.