I am presently teaching someone the exposure triangle.....using fully manual mode and spot metering with a D90
Upto meeting me he has always shot in either fully auto or Tv / Av priority and often not been happy with his shots.
Not unlike myself he is not a fan of pp so wishes to keep that side to a minimum so therefore is looking to capture the result he wants in camera.
eg: Yesterday he shot a flower in Av priority whilst with me and although the cameras metering system supposedly exposed correctly and the shot was perfectly acceptable, he had a personal taste for the flower to be darker..
In an attempt to get the result he wanted, he stopped the aperture down an additional stop....I knew what result he would get but for his learning I let him take the shot.
His reaction was the same that the flower was too exposed for his tastes but he didn't understand why as he had stopped down further.
Once explained that by stopping down the additional stop, the camera compensated by decreasing the shutter by one stop and therefore threw out a similar exposure result.
We put the camera to manual and hey presto, he was able to obtain the result he wanted.
Not only does learning fully manual mode help you to understand the effects of aperture, shutter etc etc but also it enables you to be creative and get the exact result that you want.
This is not to say that the semi auto modes don't have a place........