In the future I plan on querying the values of the analog pins remotely from the internet via a REST API. Without physical access to the board, I would like the board to return zero values for those pins which aren't connected to anything.
Others may rearrange, add, or subtract sensors to different pins. I'm hoping there is a way to avoid having them also remember to attach unused pins to common ground. It would be easier to just have them plug sensors in and not have to worry about anything else.