Well, I'm not really a 4E fan, but my two cents:
Copper dragons actually make more sense as Unaligned, given that they were "Chaotic Good (Neutral)" in previous editions. But gold and silver dragons as paragons of goodness and wisdom are part of the D&D tradition, stretching back over thirty years at this point. Granted, Basic didn't have silvers, but even there, Golds were the iconic Lawful dragon.
Does that mean it can't be changed? No, but with that much history behind it, and the fact that this iconic role is key to Dragonlance, shows up in the D&D cartoon, Forgotten Realms, and other places, and even has a pair of iconic deities (Bahamut vs. Tiamat) dedicated to serving as its pinnacle, the burden of proof lies on those who would change it, IMO.
So, what do Unaligned gold and silver dragons add to the game? It darkens the setting a little by reducing the number of powerful and iconic good creatures, adds some more creatures to the types that typical PC groups can fight, and places dragons as a whole firmly on the "Indifferent to Hostile" end of the spectrum of creature types from a civilized point of view. Whether this outweighs the loss of their classic role is largely a subjective decision. Thinking about it, I find the third element most convincing of the reasons for the changeover, but I've generally held the view that dragons often work better as symbols of primal evil than as 'just another creature type.' My Dragonlance Anti-Canon has metallics that are Unaligned at
best, but that's a decidedly subvervise take on Krynn.