Digital has its own demands as does color negative film, color chrome film and B&W film. And digital can further be qualified as to exposing for a RAW file or for a JPG and TIFF files. Knowing your media and how it responds to light is as essential as understanding the nuances of a given light meter. And it matters not if the meter is in a camera or separately held in the hand, whether the meter interfaces automatically with the camera's exposure controls or if such interface is manually set such as for a camera without a meter.
And then there is the essential consideration / parameter of processing. Some processes (E6 for example) leave very little leeway for recovering from exposure error. The RAW convertor / processor in Adobe PhotoShop CS3 (and 4 & 5) together with a quality RAW file (DNG, NEF, etc.) exposed well within the ISO limits from a camera such as a D3, D700, M8.2 or M9 offers a substantial exposure flexibility given the very real latitude of the process. Other elaborations can be stated, but there really isn't any point.
I use auto white balance and auto exposure because I have come to understand through using my cameras and computer / software what the limitations are and aren't, what I can and can't do. I learned exposure exposing and processing B&W film, later on I started shooting chromes and refined my exposure technique with Kodachrome. It was a major step towards exposure recovery when my job switched from E6 chromes to C41 color neg films as there was some opportunity for color correction after the fact (ie, after the exposure and film processing.)
The early days of digital was not unlike shooting chromes. Cameras could manage white balance but AWB was chancey so everyone carried gray cards and set custom white balances for every situation. Shooting JPG and TIFF files also demanded very careful exposure with serious attention given to not over exposing and blowing out a highlight. The move to PhotoShop CS and shooting RAW files provided more latitude than that available with the C41 color negative process.
When you know your camera, media and process you should understand whether or not AWB and/or AE can be trusted. It isn't ego or arrogance, it is experience. If a mistake is made it is an opportunity to learn, not to ignore and perhaps repeat the mistake some other time.
A camera is a tool. So is a saw. I own power saws and hand saws and understand when it is appropriate to use each. I also own two different incident hand held meters and a hand held one degree spot meter. There are times appropriate for using them as well. Likewise I have meters built into each of my digital cameras and have endeavored to learn those meters so as to eliminate guesswork.
