There is a lot of differentiation being stated between artificial studio lighting and natural lighting. However, while the physical facts of each lead to certain qualities and situations that become commonly associated with each, in the end light is light, and metering theory is the same for both.
No matter where you are, your light can either be direct (e.g. sun or lamp on face), reflected (e.g. bounce back of light from the environment and sky, bounce umbrella, or bounce card), diffused direct (overcast day or softbox), or diffused reflected (not super common in nature, but the flattest light you can get, and also the hardest to work with IMO; examples would be a bounce umbrella shot through diffusion - or a "beauty dish" when used with the diffusion over the reflector - or reflected sunlight from the environment and sky coming through curtains).
In almost every single photographic situation, you are going to have at least two of these types of light in the picture, illuminating different parts of the subject with different qualities and different intensities. (Exceptions would be things like a single light studio setup with steps taken to eliminate all bounce, or shooting someone illuminated by a "stream" of light coming into a totally dark building that provides no reflection of the light whatsoever.) There are almost always a main light and a fill light, whether you are outdoors or not. A subject shot on an overcast day is primarily illuminated by direct diffused light from the clouds – the world's largest softbox. But it is also being illuminated by reflected light from everything around it: ground, sky, buildings, trees, etc. A subject shot in open shade is primarily being lit by reflected sky light. But it is also being lit by that sky light bouncing off of the ground or other nearby objects. A subject shot in hard, direct sun is not only being illuminated by the sun, but by reflected light from the surroundings and the sky. It is just the same in studio, only it is more obvious to the untrained eye; the fact that multiple sources are illuminating the subject is painfully obvious in the studio. But this doesn't mean that the same thing is not going on in nature.
So, the statement that metering the main light is only useful in studio does not make sense. There are multiple sources of illumination, usually of some difference in intensity, in nearly every shot. You just have to realize it even though it is harder to do than in studio.
The other thing is that I have heard it stated that an incident meter averages what the camera sees. But the meter is completely independent of the camera; that is why you are using it and why it is such a great tool! An incident meter averages what it is pointed at, not what the camera sees. There are 180 degrees around its dome; you choose how to best use those 180 degrees depending on the situation.
Don't get me wrong; it does average. But only what it is pointed at; it doesn't have to be used to average main and fill all the time. Do you want to average all the light coming from the main source? Do you want to average all the light coming from a secondary source? Or do you want to split the difference? The answer is that it depends. Incident meters are simply used to measure light falling onto the meter's dome, wherever you choose to point it, not just to meter the light falling on a plane parallel to the film plane. Aside from making sure that what you are metering is visible in the frame (for purposes of practicing common sense) this has nothing to do with that your camera sees.
Not wanting to average the whole picture based on the light falling on a subject plane that runs parallel to the film is part of the reason why we use incident meters. Yet many feel that doing just this is going to benefit them in every situation. It just doesn't make sense to take a tool capable of such precise and creative control and use it in such a loose and broad manner as a matter of course. It kind of negates a lot of the purpose of using the tool in the first place. It's like getting a Toyota Prius and driving it like a maniac just because it won't cost you that much in gas. You get that car to use as little gas as possible, not to cheapen the costs of your aggressive driving. Doing the latter is to waste the capabilities of the car, and to make things worse for yourself.