The closest you're going to find to a coherent argument in favour of Slashing/Piercing/Blunt debuffs is the idea some people seem to have that it's supremely important to the game's sense of depth and challenge that you open every fight, every add spawn, and every post-downtime reengagement with 2-3 GCDs where you're missing 10% of your damage.

Every other argument is inconsistent at best, and more frequently, just completely incoherent. There's just really no other defence - the most likely explanation for their current incarnation is that SE flat out did not think things through.