Spirit shields are in an entirely different category, and are well balanced with the rest of the game. If you're playing untwinked, it would be pretty bad planning to assume you're even going to find a four socket shield at all. Even if you do, it'll be when you're mostly finished with the game. And of course the strength requirements are huge.I don't know if I want to make a Spirit shield now. It's just too big an advantage.
At any rate, I don't think Spirit or Insight (I agree the meditation is borderline useless-- chars that need mana don't get enough and chars that have mana don't... need... mana...) are that outrageous compared to some staples like Rhyme, Steel, Duress, Crescent Moon, or Strength. Runewords in general are quite useful.