When I am digging, I usually want to dig out a rectangular area.  The area might be a big wide room, or a long narrow tunnel.

My suggestion is to add a “drag select dig mode”.

This mode could be activated by holding down a particular key and then dragging the mouse to select a rectangle.

An additional option would be to allow “drag select dig mode” to be set to be the default dig mode.  (Meaning no key would be held down.)  This option would probably be exposed via a checkbox, similar to the current configuration checkbox for “Right Click”.  (Something like: “Left Click: [ ] Brush select mode  [ ] Drag select mode”.)

Another option would be to also implement a “drag select shield mode”, allowing shields to be created in arbitrary rectangular areas.

These “drag select” modes would (obviously) be an alternative to the current brush-based selection mode.