Obviously, martial law is only necessary in cases of extreme chaos. Normal law and order are appropriate for most places at most times. Giving the military power over civilian society is usually a dangerous option. The military should only have control of a country as a last resort after extreme disasters.
Martial law is just an excuse to permit governments which already routinely trample on the civil rights of their citizens even more so than they ordinarily do. This all takes place under the guise of so called increased safety. We should not trade freedom for a false sense of security.