photoflyer, intensity/ratio is just one of the 4 characteristics of light that can be controlled when you bring lights "available" to you to the party. In many if not most cases, existing light can be improved by tweaking the direction, diffusion, intensity/ratio and color of the light in the scene. Yes, there are some situations where ambient is great or the subject can be moved to good light or positioned to better use ambient, but having the ability to control light allows you to use backgrounds not otherwise useable, to control bg brightness, to place complimentary shadows/highlights on the subject. We work in light so learning to harness it will make us better photographers. Having the option to go with ambient, to modify it or replace it gives all kinds of options. You are right, high iso capability of modern digital lets us photo in low light, but low light isn't necessarily good light. It is often too hard or from a bad direction or has inappropriate ratios or is the wrong balance with the background. Having control allows you to create your style, your interpretation of a scene/shot, not just take the shot everyone takes, make the shot as you envision it.