It never stops, does it? Last night brought the news that Mary Travers of Peter, Paul and Mary had become, at age 72, yet another beloved entertainer gone too soon. Not a complete surprise — Travers was diagnosed with leukemia in 2004 — but very sad nonetheless. Peter, Paul and Mary played a crucial role in helping the folk-music scene become a mass popular movement in the early 1960s. They couldn’t have done it without Mary Travers’ clear, expressive vocals. A gifted interpreter of others’ songs, she was the principal reason why the trio’s covers of Pete Seeger’s “If I...