Excellently said Shad0w.
fas723, having a Jinx thrown in the mix has saved my Mage and my creatures on more than one occasion. The same could be said for Decoy. Granted both have a negative cost to you in terms of mana (granted Decoy returns mana to you for a net loss of 0 mana), the player, but what you are in essence doing is prolonging the game just long enough for the flow of battle to turn in your favor, and you've cost your opponent his action marker, or quick cast marker.
A good meta-game analysis that I heard Shad0w say at the demo Tournament this passed Gen Con was (and I am paraphrasing), "Always watch the tempo of the game. If your opponent is casting a lot of spell and has no creatures, work to stop his casting. If he is summoning a lot of creatures, work to stop the creatures."