Jonathan K. Baum wasn’t quite sold on becoming a law firm partner when he got the chance 20 years ago. He liked pro bono work too much. So after passing on an opportunity to become a partner at Sidley, Austin LLP in 1993, he pitched a position to Katten, Muchin, Rosenman LLP that he and many other lawyers said became the first of its kind — director of pro bono services. With that title he would have no billable-hour requirements. Instead, he would make it easier for others to do what he would have …