I have to admit, I cringe every time I see a wedding or portrait photographer advertise that they are a "natural light photographer. " It's as if they have transcended every other photographer and they don't need to use artificial light, they are purer and more attuned to the natural world, but what I'm really thinking, is that they don't know how to use flash. I get it, if you're shooting natural light, you can lighten your kit, not having any big bulky speed-lights or mono-lights to contend with, no softboxes, no assistants, just you, a camera and a couple of lenses. But, are you really giving the client what they think they're paying for? C'mon you can do a lot with even an on-camera mounted flash. If the conditions are right...natural light, if not flash goes a long way to making an image pop.
Don't get me wrong, I love shooting natural light when I can, and I think using natural light is great when you can do it, but it kinda limits you to shooting during the golden hours at sunrise or sunset or an overcast day or a well-lit interior. Maybe it's because I learned photography shooting film when your sensitivity pretty much maxed out at ISO 800. Sure, there was 1600 and 3200 (b&w) film, but for practical matters 800 sensitivity was it. There were no "natural light" wedding or portrait photographers in the modern era except maybe for photographer Jeff Ascough who was using very expensive Leica cameras (3 of them), and lenses to include a 50mm f1 Noctilux, a 50mm f2 Summicron, 35mm f1.4 Summilux ASPH, 90mm f2 Summicron ASPH, and a 21mm f2.8 ASPH, some of the priciest glass on the planet. Jeff is home based in the UK where cloud cover is prominent and even Jeff would revert to flash when he needed it. Mind you, this is when he was shooting film, but the last I heard he had migrated to Canon digital cameras and is using more flash than he has in the past.
Modern digital cameras can shoot to 6400 sensitivity (ISO), and you can push them to 51,000 and beyond so it's no particular skill set to shoot in low light anymore even with zoom lenses. The truly gifted natural light photographers learn to read the light and position themselves to take advantage of the light, but when the light isn't right or it just gets too dark or your subject is backlit, knowing how to use artificial light/flash is a valuable skill. I contend that most people with a decent eye can get comfortable enough with a digital camera to turn out pro-grade work in a couple of months, but it's sort of like learning to ride a motorcycle. It's pretty easy to get comfortable with riding a bike, until you get yourself in a situation your not equipped to deal with and you crash. Hopefully, you don't kill yourself and you live to ride another day. Hopefully, as a photographer, you don't get a paid photography gig, where you're over your head, and the natural light thing just won't work for you.
I know that many of the great photographers, before the modern era shot natural light. As Frank Van Riper, in one of his many columns said "Many of my favorite photographers – Robert Frank Garry Winogrand, Cartier-Bresson, and Gianni Berengo Gardin, to name just four – always use or used available light. [And they were the photojournalists. The landscape masters like Ansel Adams, Paul Caponigro, and John Sexton also worked this way, only in larger format.]" But, that was years ago, when carrying portable light with you was much more difficult than it is today.
When I travel, I usually don't carry a flash or any artificial light source, but that's shooting for me, not commercially. If I were shooting for a client I would always have at the very minimum a small flash...just in case.
My problem is not so much with people who shoot natural light, (I do it quite frequently), but with photographers that brag and advertise it as if it were some special category of photography. To me, it's telling the world that you haven't quite figured out the photography but you're advertising you're a professional. News Flash: If you haven't figured out how to use artificial light...you're not a professional photographer. Take a class or two, shoot natural when you can, but know when to use flash. You'll be glad you did.