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Dark roast isn't necessarily bad, quite a lot of people prefer the taste, especially if they're adding milk or other things to their drink.

But a dark roast is easier to produce, and easier to produce consistently. If you take high quality beans, and low quality beans, and roast them both to a dark level (let's say "French" or "Italian" roast), they're going to taste approximately the same. Therefore if you're producing coffee at a larger scale and want to save money, you can use cheaper beans and roast them dark to mask the imperfections that come with the cheaper beans.

There are some truly incredible coffees out there and a well-executed light or light-medium roast will bring out those flavors beautifully, but the quality starts at the coffee farm. You can't light roast a low quality coffee bean and expect those same excellent flavors.