Add Pittsburgh to the list of cities welcoming the Rockettes to town for the holidays. And as usual, the arrival of the touring Radio City show is being greeted by both enthusiasm and trepidation. On the plus side, the show could pump as much as $25 million a year into Pittsburgh’s hospitality industry. But for local arts groups, the Rockettes represent a serious threat to their own holiday-themed programs, and thus to their overall bottom lines.