Shadow as a "power source"?

[mneme]

I have to agree (though it doesn't bother me enough to hack my own game, should I run one) that Shadow is kinda silly as a power source. Yeah, yeah; shadow means you've taken a bit of shadowstuff into your own soul -- Assassins have a bit of Nether; Vampires are pure Necro. But putting a specific element / planar source on the same level as mechanisms like Martial, Psionic, Arcane, Primal, and Divine (ie power from: your body, your soul, your mind, this plane, other planes) seems unnecessary. If one wanted to get rid of Shadow while keeping the Shadow Assassin and Vampire, it's easy enough to say that the Shadow Assassin is Psionic, while the Vampire is Divine.