When regular riders gripe about the MBTA, it’s rarely because the system is too warm, too personal, or too brimming with holiday cheer. So it’s gratifying that T officials let a dispatcher off with nothing more than a light reprimand after he programmed electronic message boards at Park Street station to display the lyrics to “Deck the Halls.’’ If anything, many riders’ spirits rose as digital “fas’’ and “las’’ brightened the subway platform, and as an affectless electronic voice read along. At worst, it was a diverting prank. According to one witness, “As far as PR goes, it’s probably one of the best things they’ve ever done.’’