"If you use a flash, you can always set exposure compensation down on the camera(1/3 - 2/3 stop or even more) and dial up the flash to darken the background"

I think this is a good approach for some shots. However, you have to be very cafeful not to over do it or your shots will start to make the subject look like they are in front of a backdrop instead of a real outdoor setting.

white balance is critical to insure that your subject is lit with the similar available or ambient light. . . If you hit the subject hard with "white, or blue" light, and the ambient light is shade or cloud light than the contrast between the two willbe obvious.

Hence the Fong diffuser, which I like but it too has limits, especially if you want to use longer lenses to bring the background closer. . .

Here's an example of compression (those mountains are a bout 20 miles away) and lighting that did not work. The light on the subject is too cool, it's a flash that's too white as compared to the background. I would have been better off to lower the flash a bit and decrease the shutter speed a bit too.

Take care,