Other than to simply say you did it, there's really no reason to turn on all of the debuff skulls on Legendary. Sure, for some missions, it would be needed in order to match the high scores (say missions 1, 3, 8, etc.), but generally only 1-3 skulls need to be on in order score higher than anyone else. The modifiers from combat and time bonuses far outweigh the bonuses provided by the debuff skulls; it's hugely diminishing to turn on all of the skulls to cripple you enough that you can't complete the mission quickly enough to get the max, or near max, time bonus modifier.
Certain missions, like missions 4, 6, 7.. turning on all of the debuff skulls would certainly cost you more points in lost time than it would add in bonuses. I know I could grind through all of the missions with these skulls on (with inferior scores against fewer skulls); I find it more challenging to only turn on enough skulls so that getting 10x and 5x modifiers is still just barely feasible.