All I can figure is that with a place like, say, Starbucks, it's a consistency thing. Even their "light" roasts are pretty dark, and their medium is "burnt to a crisp", but if you want to put 25,000,000 12oz bags of coffee on shelves and for it all to be "the same" (all "Pike Place", say) then I guess burning all the character out of whatever beans you got ahold of is one way to do it, versus sourcing all the beans from the same small set of farms in the same country in a single season or whatever.
I figure it's a similar story for most of the other big national or multinational coffee roasters, which is why it's almost impossible to find those correctly-roasted, delicate, tea-like coffees from anything but tiny roasters. Those places, two of their "medium roasts" may taste wildly different and if their supply of the beans for one variety dries up, that one's gone unless they can find a way to get more of it. Dunkin or whoever just want to always be able to have a bag with the same name on it on the shelf, and for it to always taste the same, even if that same-taste is not very good.
The espresso-loving nation of Italy universally roasts their coffee way too dark. I think it’s just a holdover from when coffee was first introduced to Europe.
I prefer medium roasts over light roasts, which isn't quite the same thing as dark roasts, but it's because it tends to get rid of fruity and acidic flavours and lets the richer flavours come to the front: brown sugar, chocolate, malt, nuts, spices, etc. which I prefer in milk drinks.
No idea what the dark roast people are after, but maybe it's the same sort of thing.
