Currently, even if "Show cumulative grades" is unchecked in the gradebook, assignment type "cumulative" grades still show.  This makes it trivial to calculate the cumulative grades and, in cases where there is only one assignment type, the cumulative grade is shown through the "assignment type cumulative".

Basically, cumulative grades are not hidden when the checkbox is unchecked.  Students still can find their cumulative grades (albeit not under the "cumulative grades" heading).