To the previous good responses I'll add that I do use an incident light meter (a Sekonic 758R) in several situations.
1. I am occasionally commissioned to photograph 2D and 3D art works.  In these circumstances color and exposure accuracy are critical.  The reflected meter built into a camera can be fooled by some of these subjects.
2. When working with strobes triggered by radio signals (i.e. Pocket Wizards) the Sekonic can be set to simultaneously trigger the strobes and take an incident reading which I then transfer to the camera.  This is indispensable.
3. When doing some tabletop work, even with continuous light, it is sometimes much more accurate to use an incident meter and/or a handheld spot meter.
Reflected metering systems in today's digital cameras are, in the main, excellent.  You may never need to use a handheld meter, particularly since you can evaluate exposures immediately and for free in a dslr.  But I personally like to have a good handheld in my toolbox.