Practicing off live pitching has it's benefits (vs machine) but the context under which it takes place is important. In my experience a lot of live pitch BP does not take place with the context of an at bat, most of the time it's pitchers throwing fastballs and batters taking swings. Live BP is most effective when it mimics real game at bats, someone has to call balls and strikes, and pitcher has to have the freedom to use her full arsenal to strike out the hitter.