Preacher curls leads to over-shortening and over lengthening of all the muscles that support your shoulders while lifting, this is NOT GOOD! You over lengthen your rotator cuff by moving your shoulders forward. Moving your shoulders forward also shortens your pecs. This will also cause your lats to lengthen. All this leaves you with NO support for your shoulder girdle. Your shoulders are stable when they’re down and back, the opposite of the preacher curl position, in a natural position.
Now, this may not seem like a big deal. You don’t spend all day doing preacher curls, but over time it can catch up with you, especially when you consider most of us are in this position all day at work. Think about it, all day in this position hunched over at your computer. Then you go to the gym do preacher curls, it’s just reinforcing bad posture while under load, increasing your risk of injury.