Yes, You Need Flash!
As I look at the majority of outdoor photography today I see a lot of photos taken with existing light. There is a good reason for this. Under the right conditions natural light looks wonderful.
Unfortunately, the times where the existing or "natural light" is just right are limited. They are limited to the time of day or actual location of your subject.
What happens when you have to take photos during a time or location that the existing lighting is just plain bad? You need flash.
Taking a photo of the house under existing light would give me one of two choices. I could expose for the house and let the sky and windows go completely white. I could expose for the sky and let the house go completely dark.
By exposing for the sky I could then add flash to light the house. The flash was set at one f/stop underexposed so as not to "over expose" the house. The photo you see is without any manipulation. Later in Photoshop I can darken the grass in the foreground so it is not overexposed and the attention goes to the house.
The photo of the bride was taken with existing or "natural" window light. It's not bad but for me the shadows are a little too dark. I can lighten it and loose the highlights. I can go into photoshop and lighten the shadows there. Or, I can just add a little bit of flash and save myself editing time.
I set the flash to 1 f/stop under so as not to kill the shadow but to lighten it a bit. Now if your taste is different you would adjust the setting to whatever you like.
You can make the argument that it could also be fixed will a reflector. This is valid except now you need an assistant or a stand as well. Try using a reflector outside and a gust of wind comes along.
There are many times when natural light will do but for the rest of the time, yes, you need flash.