DRK have the highest threat because they are likely not seen 'using' that combo as often. Because of their mp issues, they will likely spam soul/siphon much more often, meaning that they need their emnity generators to be stronger than paladin or warrior whom generally 'dip' into other combos to put up debuffs and then return to their emnity combo (which was also their damage combo).

DRK is different, their damage combo is also dark arts+soul eater anyways.