Rock of Ages, this week’s hair metal spandex singalong, asks a basic question: what’s the point of a musical? More specifically, it asks a pair of underlying questions about musicals: is enjoyment a worthy artistic goal? Is sentimental simplification acceptable in the name of fantasy and fun? On one level Rock of Ages does to the metal years of the late eighties no more or less than what Singin' in the Rain did to the twenties or Grease to the