Actually, it does prevent both sides, as well as absorb quanta. I guess it needs to be specified. It's not exactly fair if you play it otherwise. In that battle, if both players had that card, the one who played it first would win. This card would be best with creature spam decks, or rush decks if you're trying to slow down the opponent. The original idea blocked both spells and permanents, but since that would stop pillars, it was decided that was too much.
