Old DC power to Music Hall theater?

[thomprod]

I worked briefly on the stage crew for Michigan Opera Theatre when they re-opened the Music Hall in November 1971 and again in the Fall of 1972. I remember that the old, exisiting stage lighting board ran on DC power, still supplied to the building by Detroit Edison. Many power outlets in the building provided only DC power and had to be marked "no motors". Was this a widespread practice in Detroit theaters? Why was this considered more economical or efficient than AC power? Does anyone have knowledge of how widespread the DC power distribution in the downtown area was and when it ended?