There's possible application in drainage workflows - both waste fixture unit calculations as well as stormwater calculations could benefit from having a slope parameter to assist driving calculated parameters that determine flow rate or pipe size.
The simple answer to the lack of scheduling feature is that they 'probably' could not or did not want to reconcile the case of vertical pipes. The storage type of the slope parameter is 'number' but this means that vertical pipes are Undefined - I wonder how the Calculated Parameter feature interacts (if at all) with undefined values. Regardless, do you want the pipe schedule to say 'Undefined'? Revit would have to specifically identify that the Slope parameter, when Undefined, should read 'Vertical' in schedules...that frankly sounds like a lot of work that is easier to push onto the users, and @iainsavage 's link to past work shows the user solution.
