I prefer flash.
So, which one is better? Ethical?
For me, both are.
Shooting with available light, to some is the only way to go.
Not for me, I am shooting both. It really depends on how I like my photos to look.
I will try to differentiate the two by posting sample photos.
The photo above is shot using the ambient light only. Purist style, no fill flash.
The sky is a little gloomy, light is nice and diffused, there is no need for me to use flash.
This gives a more airy look with nice natural colors.
The above photo is taken seconds after the first photo.
I dialed the exposure down a bit, then lighted Lyn with a single flash at camera left.
This gives a more subdued look and gives more focus on the subject.
Also, notice that the colors of the background are different now?
It is really up to you
For me, using both ways are okay. One light is natural, the other is man-made.
Both looks good too.
It really depends on you and the feeling that you want to convey on your photographs.
To stick to only one way is limiting yourself.
To be a 100% purist is losing the opportunity to be more creative.
To be a 100% strobist is failing to take advantage of a beautiful natural light.
To be using both is great. You will have as many possibilities of conveying your messages through your photographs.
Learn how to use both.