doctorhook said:
"Because Gary said so."
That doesn't make a very good reason for anything, ever, does it?
If you think that's the only reason, I've got 30 years of fun D&D with at least one good-aligned metallic dragon in them who would be shocked to find out that they weren't really as much fun as they thought they were.
More relevantly:
SkyOdin said:
Personally, I prefer the new paradigm where both Chromatic and Metallic dragons can be either Good or Evil. Why would every Gold Dragon be good and every Red Dragon be evil anyways?
The biggest reason is probably one of fantasy storytelling. The evil Red dragon and the good Gold dragon are really quite archetypal; and the Good Dragon/Evil Dragon dichotomy exists far outside of D&D. D&D, in order to be a good narrative game, should include the rules for using these narrative archetypes in the game. That doesn't necessarily mean statblocks for fighting gold dragons, but that does mean rules for what they are and what they do when you put them in a game, as good, benevolent, yet perhaps adversarial, creatures. That does mean that the good dragon and the evil dragon both should have their supported place in D&D.
The second reason supports the first, but is also distinct from it: the idea that not every monstrous creature is there to fight the PC's is key to having a variety of different types of challenges in the game. Strong support for non-combat narrative challenges is key for a game that is diverse, especially a game that is about, to a certain extent, storytelling. The gold dragon, as an archetypically Good creature, was an opportunity to showcase this concept.
4e's current dominant design paradigm regards the narrative reason and the "not every challenge is combat" reason as fairly worthless, but my own games, and I'd wager the games of a lot of players, would be made more robust by including at least one iconic evil and one iconic good dragon, even if the rest of them were up for moral grabs.
IMO, this is related to the broader issue of 4e's functional ruleset vs. traditional D&D's more descriptive ruleset, but those two reasons are very important to my games, at least.