I once worked for a place night shift boning out lamb fours(Shoulders of lamb) and flanks for kebab meat. Personally from a professional point of view i found they were very diligent in the quality, and insisted on near religious trimming of anything that would make it unpleasant, as in bone fragments, ligaments, gristle, skin.
As to sausages in a retail butchers. Theres two items that must be top quality. Mince and sausage. If either are rubbish the shop will be shut in under a year. When you break down carcasses, you have the main cuts, roasting, stewing, frying etc, and a shed load of trim.
Now while in wholesale the price of the entire side of beef might be £1.20/pound, this means fillet, shoulder etc is £1.20/pound, but also bone, fat, connective tissue, gristle etc is also £1.20/pound.
So to remain in business you really need to be able to sell that trim, and in that its lean meat trim goes to mince, and fatty/meat trim goes to sausages. What you throw out needs to me pretty much gristly bits and bone only, the rest you really need to sell or you will not remain financially viable as a business.
If the sausage or mince is fatty, then the customers wont buy it, and if you cannot sell the trim, then you lose a substantial part of the side of beef you’ve bought.