When shooting digital, there is absolutely no reason not to use manual.
Please let me explain.
The metering of the camera works by taking a reflected reading of subjects that may be black, white, grey or a mixture of colors and tones. This means it is not able to give an objective exposure. As such, within a scene, like a bedroom room, the exposure of each shot might be different.
I prefer to work by setting the exposure based on the light source and then compensatating mentally when necessary.For years, photographers have shot outdoors with the sunny 16 rule, increasing exposure when under shade.
The truth is that the light in a given scene really doesn't change that much. By using P or A, the meter might be fooled into using a slow shutter - which means motion blur.
I was shooting on a movie set. Had ISO 1000, f2.8 and 250. I let the images underexpose a little, and push in post (RAW). I find that this is better than having the correct exposure but blurred pictures. When shooting digital, it is better to be slightly underexposed rather than over exposed. This is opposite to film.
The logic to using Manual mode is for the photographer to be constantly thinking and being aware of what is happening around him/her. With digital, you can always do a test shot to determine exposure, and make adjustments mentally when you frame a darker or lighter scene. A good photographer should always know where the ballpark of exposure should be. He should already set his camera according to the scene just like how our eye iris constrict and dilate when we enter rooms of different brightness.
There are advantages to the Auto ISO and P mode but in my opinion, the photographer's judgement is more accurate than that of the reflected meter.
If you have a incident meter, even better ! Check the weather when you arrive at the scene.
The concept to grab is : expose for the light, compensate for shadows when necessary.
But avoid exposing entirely for shadows. We measure light. Not darkness.
