Ha, that originates from me having converted some amenity=drinking_water to waterway=stream somewher in Argentina and our discussion that ensued.
My comments:
1)the names are too long!
2) For the example given, that is a spring, we already have Tag:natural=spring - OpenStreetMap Wiki (and it has tags for all the properties maybe apart from capacity)
3) for sources that are not spring, there is waterway=stream. I am not sure there is a way to indicate if the water is drinkable but I also think that would be quite subjective, who is to judge that? I will drink a lot of water that others will zealously treat and i am usually fine.
I think the motivation is to have something in the database that one can search for, so in a way it would mimic the functional/spatial distinction we have for highways and waterways. I think it is a bit superfluous, but I am curious what others think.