Actually, in Prop 8 they heard the case on the technicality (just not on merits), and ruled on the technicality to vacate the 9th Circuit Court of Appeals' decision. This threw things back to Judge Walker's original federal court decision, which does strike down Prop 8.

If they had refused to hear the case, they would have dismissed the case as improperly granted, and the 9th Circuit Court ruling would have stood.