Newer Leica can do aperture priority. Just set the shutter speed dial to the "A" position.
I usually use Aperture priority with Auto ISO, Max ISO to HI.2 (highest my camera can go) and shutter speed to whatever minimum I think its appropiate.
When I have a portrait (single person), I use maximum aperture.
When I have a group or other larger subject, I raise the aperture.
When I try to photograph a landscape or shoot with my weaker lenses (*), I use f/8 or f/11.
THis technique means, though, that I have to do a lot of menu surfing whenever I want to use different minimum shutter speeds.
When I want time effects, I use shutter priority. I am thinking of getting a variable ND filter for that.
Oh, and I have no qualms to simply set the mode to "Auto" whenever I feel that this will give me the shot.
(*) This refers to my kit lens and to my telezoom in the longer focal lengths.