Most all cameras these days have a built in light meter. Do you need one outside the camera to compose a great image? Well, it depends.

On a digital SLR (at least the Canon cameras I own) you have 4 metering options with relation to utilizing the light meter in the camera:

Evaluative metering (full auto)

Partial Metering (meters light in circle you see in the viewfinder)

Spot Metering (meters light on a certain spot in the viewfinder) - not all cameras have this and the options are dependent on the camera

Center Weighted Average Metering (the metering is taken at the center, then averaged throughout the entire scene).

Each should be used for the appropriate situation. In some higher end cameras, you can associate the focusing point with the light metering point ...