We don't need more pointless placeholders that need to be removed and redone soon enough anyway.
If you just throw it in, dwarves would probably interrupt their work every time they sneeze and then you have to fix that and all the other bugs that result. It's not worth coding time to half-ass it.
"Pointless" -well, besides having a perfectly decent, fun method for the spread of infection diseases- a fairly accurate one at that, based on expelled germs, which could be significantly counteracted by cleaning- we could also begin preliminary testing of how DF will work with diseases, so that when and if a more complicated system is needed, some testing will already be done.
Secondly, we already *have* the effects I described, in terms of what dwarves actually do or get interrupted by- all I'm proposing is let the existing conditions contribute to the spread of disease, and add in a gaseous one.
As for programming time and bugs- the whole reason I proposed it this way is that it would be very easy to code. The vast majority of what's needed already exists in game, with the current poisons selection. The *only change that would really be needed would be to copy-and-paste existing "cough blood" code, and cause it to expel a custom material created in the RAWS instead. From there, existing code can handle everything.
Finally, the only support you've given for this being half-assed is that it can't handle waterbourne infections, which, as stated, would require additional code for mixing liquids (something Toady has mentioned as difficult in many of the talks). Once such becomes available, the system I proposed could certainly work with that- if bile is expelled into a water source it mixes in, and dwarves exposed to the water function as if exposed to the bile, by some mechanism. So, I guess I retract my previous statement- this doesn't need to be a placeholder at all, it could in fact be a perfectly functional long-term system.