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No I have never used it in a dry cured sausage where fat smearing could easily coat the meat and casings and prevent drying. Hard Back fat also cuts to more defined pieces that adds to eye appeal, and has a much nicer mouth feel on a raw(uncooked) product, as opposed to soft and mushy fat. Using in fresh sausages like you mentioned is fine, as long as the fat is frozen or near frozen when grinding and mixing. Its also great to use in emulsified sausages.
Another Toldra book that may interest you is here: https://www.academia.edu/8548143/handbook_of_fermented_meat_and_poultry
I am not saying you cant use soft fat but if you have a choice….