The only use of scrap metal i've seen really usefull in the long term is for the little lights on the ground. You can spam them, I have around 700+ lights on my world to guide me with my vehicules.
As for rocks.. Unless pavement becomes useful (I mean, if you can at least build things on it) and it takes lots and lots of rocks to build, I don't see how @cairn4 would be able to utilize it :D
Having scrap get smelted into metal plates would be cool, I would probably make it less efficient, like 2 or 4 scrap = 1 metal. The problem I just realized though is that having multiple recipes for the same item would be tricky to convey to the player right now. Would have to think about how the crafting UI could reflect that. Maybe it could even be done at a separate "Recycling" base module.