Smashing, Lethal, and Energy are the three most common damage types, and the vast majority of incoming attacks tag at least one of the three. Scorpion Shield offers only slightly lower defense against S/L than its elemental counterparts and in exchange gives you a chunk of energy defense, making it objectively the strongest defense toggle available to non-armored classes.
Also, if you care about your character's appearance, Scorpion Shield is one of the least obtrusive armor toggles.
Vectored defense sounds easier to slot for in theory, but more sets offer bigger typed defense bonuses, and it's far easier to get a typed-defense baseline from powers.