One of the biggest problems in volume control we have right now is that we have too many volume controls in line. The app might have one, the device has a couple, the hardware might have even more. It is very confusing to the user to find his way through this jungle and figure out which one is the one that is responsible for the low volume of the output.
The fix for this is to coalesce all those volume controls at a single place which should naturally be the sound server. If applications then want to expose a volume control slider they should expose the one maintained by the sound server -- instead that everyone implements its own.
With the "flat volumes" feature of upcoming PA 0.9.13 we even will be able to coalesce per-stream and per-device volume into one.
In summary: requesting that each app should implement its own volume controls goes in the wrong direction. Bad idea. Right in contrary exporting vol control to the sound server is the right answer here.