Naturalist looking flash photographs are actually very difficult to achieve, most look like studio portraits - key light, fill in or reflector, a little bounced or back light. Daylight by comparison has a vast range of colour temperatures, levels of diffusion and shadow depth to be exploited, so long as the photographer isn't on the clock in the set up.
It's not a question of evangelising one light source, more one of exploration. Studio flash can lead to highly formulaic shots that look like everyone else's idea of studio photography. A lot of contemporary fashion and editorial work is on location and in available light.