Portability and less film cost are the big factors for me. A few weeks ago I went to Balboa Park in San Diego and had 2 cameras, an SRT101 & XD11, plus a Gossen meter around my neck. One with color and the other B&W. There were a few positive comments to my liking.
If I had taken my Crown Graphic, dark cloth and tripod it would have drawn a huge crowd, as has happened in the past. Of course it's hard to focus and compose while being asked tons of questions.
So I guess discreet is another reason. Blending in more with the crowd.