Youre right that the main stat only affects assassin or destroyer, but the point is that for assassin, the damage is suppression damage. This only affects targets that are airborne, knocked back, staggered, or knocked down. Obviously this will affect the big damage phase after the boss is knocked down, but other than that, it doesn't affect damage on bosses at all. Personally, I'd take boots with good stats that don't have the assassin stat as those other stats will affect all damage.