I personally don’t use stoneflies as much as I should in the waters I guide but they allow you to really go to town on profile, accents and importantly for big, come eat me legs, let’s talk more about this...
A good stonefly imitation is my go to in deeper, rocky runs, or big pools where you need your fly to be seen. On many of my Southern waters going too big, too soon can kill an opportunity, however for me personally after presentation, bling, then weight, I want a stonefly or two in my kit.
Rubber legs on a stonefly nymph add enticing movement whereas stiff static legs push water and create an aquaplane effect, enhancing the visibility of your fly.
Often something big, visible but lightweight is just the thing along the slack, knee deep edges and the addition of deep soft weight will get them plugging the runs if needed. Otherwise I reach straight for the tungsten, you choose.