I have them, but rarely use them. Usually a solution in search of a problem. If you don't need one, they just overwork perfectly good brass which shortens useful life.
If your brass source is the gun for which you're loading you'll never need a SB die.
Sometimes individual pieces acquired via bulk brass purchases or range scrounging are found pretty stretched. Easiest way to identify them is to use something like a Wilson gauge. These pieces can be made to chamber using a SB die. Lacking a SB die some choose to run the pieces through a std sizer a couple times. The much better approach is to recognize these pieces were badly overstretched and simply cull them. When you load questionable source/stretched brass you can get a partial or complete separation.
