It depends a lot on whether the scene is changing or the light is changing. The problem with auto-exposure is that where you have a fast-changing scene with sl

It depends a lot on whether the scene is changing or the light is changing. The problem with auto-exposure is that where you have a fast-changing scene with slower-changing lighting - especially fast-changing background scenery with constant lighting, the computer will change exposure when it shouldn't. This is a scenario professional photographers probably find themselves in rather more often than amateurs - think concerts, stadium sports, etc. When the lighting changes more frequently than the scene then auto exposure can be helpful. In this sense full manual settings could be viewed as a longer lasting "exposure lock" type function in a way. It's not that the computer-assisted metering is not useful - more that you want to get the metering locked in for the actual subject then stop it from changing.