Thank you Brady,
This is by far the most parsimonious and robust script.
I would like to add one optional line of code to disable the possibility of missing values as the mode:
dt = open ("$SAMPLE_DATA\Cars.jmp");
dt2 = dt << summary(group(:Make, :R Leg));
// option to disable the possibility of missing values as modes
dt2 << Select Where ( IsMissing (:R Leg)) << Delete Rows;
dt2 << selectwhere(colmaximum(:n rows, :make) != :n rows) << Delete Rows;