In such a case it's a waste to tie up a smelter to churn out copper, tin or iron bars or a crafter for copper wire. Much better to buy these in large lots in the market in order to keep the crafters churning.
100% agree - and it's actually not only a waste, but detrimental depending on which bars you're smelting b/c some of the later, more precious metals actually require ore of the lesser valued metals. I forget the combos, but I think platinum requires aluminum, and I think gold requires tin (used to be copper, but I believe it changed), etc.
You can buy bars, but you can't buy ore ... so using it to make bars you don't need isn't wise. And copper/tin/iron bars aren't good to sell either since they're worth so little. So save that ore! Buy the bars if you have the money.