Sometimes just two states for a task “Checked / unchecked” is a bit lacking in the information it gives. Adding the other two states gives a more powerful use of the checkboxes.
I would like this feature too! But with four states: empty ( ), completed (v), discarded (x), under question (.)
Similar thread: More checkbox types - #2 by nr458h
I like this idea too. Personally I would love that clicking on a checkbox would cycle between three states: not started (unchecked), partially completed (in progress) and completed (checked).