The old version of Scorpid Sting was of dubious utility, and was rarely used (it reduced agility and strength by a static amount). The new version may deserve a second look. When used against fast hard-hitting melee mobs, it can make a significant difference to damage taken by a tank. When a mob is dealing several thousand damage per hit, an increased miss rate greatly matters.
Scorpid Sting does not apply to spell hit, and is useless against casters. However, using Scorpid Sting against Warriors and Rogues should be considered.
Scorpid Sting does not stack. If there are multiple hunters in the group or raid, designate one to use Scorpid Sting when appropriate. Additionally, it does not stack the hit reduction effect from Insect Swarm.
Scorpid Sting doesn't cause damage, and so won't break various crowd control effects (like Freezing Trap or Polymorph) unlike Serpent Sting. Since only one sting can be active at a time, this can be useful for removing Serpent Sting.