There used to be separate options for Wall and Slab Offset amounts for the Parallel Tool (now named Offset Surface Tool).
These options were very useful in that 99% of the time I only require Wall Offsets and would set the Slab Offset to 0. Currently it requires extra steps to reduce the top and bottom surfaces back to where they were prior to the offset operation. I do prefer the single field input for distance over the previous system.
Perhaps adding radio buttons for "Wall", "Slab", and "Wall and Slab" would be a simple way to implement this functionality.