A tightened bolt, or a stud for that matter, is a spring in tension. It’s true that the spring in this case tends to have a narrow elastic range, but it’s a spring that’s holding stuff together, and it’s in it’s elastic range if it’s tightened properly. Obviously one ought to use the right spring for a given job. In cast iron, which isn’t particularly strong in tension, it’s particularly important to chose the right “spring” and tension it accordingly. If there were no need for gr 2 or 5 bolts they wouldn’t make ‘em.