I like that last post. Unless the camera is doing something you don't like in an auto-exposure mode, what are you accomplishing by shooting manual?
If the frame is difficult to meter, maybe you want to override the camera's chosen setting. Maybe you're after an "artistic" effect. Maybe you're just trying to learn the effects of the different settings.
99.9999437% of my shots are shutter-priority, because I want absolute control of perceived motion in the image. Motion blur, or lack of it, is my priority, so shutter is my priority. Switching to manual would slow me down as I'd have to be consulting the meter, however briefly, before each shot.