Copper and aluminium have slightly different properties. Copper is better at drawing heat, and ally is better at radiating it away.
In other words, a copper contact block for the CPU and ally fins is the best combination.
From the reading I've done, this sounds like more of an urban myth than fact. If you have evidence to the contrary, I'd love to hear it. Also the LP53 vs L9i performance comparison kind of seals the deal for me.