Quick Guard is an extremely useful move for only a single Metal Energy. It prevents all damage done to Cobalion by attacks from Basic Pokemon. This includes EX Pokemon, since they are considered Basic. However, Cobalion can't use Quick Guard during the next turn.
This is where Revenge Blast comes in. 30 damage for 2 energy is lackluster, but this move does 30 more damage for each Prize card your opponent has taken. Getting a Revenge kill with Cobalion against an opponent's EX Pokemon means you get to take two prize cards yourself. He's very solid for a rare that costs only $0.50 for the regular printing, and $0.99 for a foil version. For a non-holo Pokemon, Cobalion #74 is a good one to have and one to definitely hold onto.