While serving as a Marine Corps MP, I had to carry my 1911 with five rounds in the magazine, hammer down, empty chamber. We had a whopping two extra mags of five rounds. I only had to chamber a round once or twice while on duty. It is a poor practice but I am certain it had to do with giving 19 year olds a badge and a pistol. We all heard the stories about guys playing quick draw and failing to keep their finger off of the trigger
I'm sure that most of the empty chamber stuff evolved from the early part of the 20th century when pistols were mostly single action. The five pound trigger is unforgiving for those with fingers on the trigger. In reading the excellent book by Fairbairn and Sykes entitled, "Shooting to Live", I was shocked to see that was exactly the condition the Shanghai police carried their pistols. The authors stressed extreme speed as a main contributor to close quarter victory but yet the pistol was in a flap holster with an empty chamber. A quote from their book, "the pistol, when carried on service, should have a charged magazine inserted but that it should never be carried with a round in the breech" They further advised that the safety should be permanently disabled and never used. The gentlemen wrote that the Shanghai police had been involved 666 armed confrontations in 12 and a half years, killing 260 criminals while losing 42 officers.
I think it's a bad practice due to the fact that one arm could be disabled. I always carried mine cocked and locked while working civilian LE. However, with practice, you could get pretty quick with it, like the Shanghai boys.