I have never seen any of these 'useless legendaries': Every legendary can pull their weight used properly, perhaps. For example, I managed to somehow get a map where, despite having 10 looms constantly tended by dwarves, the spider silk spawns faster then they can collect and weave... Who is going to do anything with all this cloth? I don't have a legendary weaver, but if I did, it would be sad that much of a weaver's time is spent walking around collecting silk... but anyway... The cloth is handed on to:
First, the Legendary Dyer. Dyamn... He's fast. Can barely keep up dye production to keep up with him using 2 specialized milling dwarves, and an army of growers. Then it goes to:
The Legendary Clothier. Egads, Take the already masterfully dyed cloth and make... Socks? A few... mostly silk bags, though. Dye production needs them. Nothin in my fort costs as much as the bags I keep them in.
In tandem, this army of collecting dwarves, growing dwarves, and one legendary clothier and one legendary dyer, produces enough to buy completely out an entire human caravan. And I am in my second year. And I am happy to say I do not use any rock to do it. Spider silk is entirely renewable. I know people don't like rock clutter, but I love it, and always need a lot of it, because I make everything else outof rock when possible... And I force a few good dwarves to become legendary miners off the bat so I can get as much of it as possible...