Festival Maariv mystery solved

I just noticed last week that the usual introductory passage before Bar'chu at Maariv (Evening Service) is omitted on the Shalosh Regalim (Pilgrimage Festivals), and asked why. This is probably obvious to my more learned readers, but my rabbi told me that "V'hu Rachum, y'chapeir avon (He is compassionate, forgives sin . . . ") is a penitential prayer, and, therefore, inappropriate for a holiday.