Answer: if things get much more complicated, the status screen will become a much more invaluable resource in keeping the fort efficient and well-stocked. Water is a fundamental part of that, and being able to check it in the status screen would be advantageous if things like:
- water necessary for booze
- other fortress mode civs
- water barrels
- sewers (unlikely, as we all know)
are fully implemented. If I have a 5x5 map with a sprawling 200 dwarf fort, it would be nice to check the status of my 5 wells and brewery water tank in one place rather than having to go look at it.
Silverionmox is suggesting a refactoring of the designation system that automatically adds the feature that OP suggests, almost. Since under that scheme stockpiles are part of room designations, and the cistern would be a room, if water is treated as a quasi item type designating a cistern is really just designating a water stockpile.
It would be a nifty alteration, if it weren't for Toady's consideration of merging stockpiles with activity zones, since they are both different ways of looking at the same thing. If that were to happen, then we might just get this feature anyway: designating a pool/pond if zones and stockpiles are the same thing would be the same thing as designating a cistern, and hopefully we would get the tracking of water amounts in the process.