Well a good DP can work satisfactorily without a monitor, he has to on film! But the image below is evidence of how a light meter cannot take into consideration the values of the surronding objects and the way the eye and brain sees the subject.

Put a light meter on square A and B and they will read the same! So a DP without a monitor will assume that square B will have the same light value to the viewer as square A. But look at the image and square B looks lighter than square A. This is a real, physcophysical effect that a DP must take into consideration when lighting. In my view this is much easier to do by refffering to a monitor.

Don't believe me? print this page and cover the squares that are adjacent to A and B

 A on set monitor gives a ringside seat of how a subject interacts with lighting the exposure and the sensor as well as revealling physcophysical effects.

There are more illustrations at http://www-bcs.mit.edu/people/adelson/checkershadow_illusion.html

Thanks to Edward H. Adelson for permission to use this image.

How does this illusion work?

The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray "paint" that belongs to the surface.

The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighboring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison.

A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the color of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible.

The "paintness" of the checks is aided by the form of the "X-junctions" formed by 4 abutting checks. This type of junction is usually a signal that all the edges should be interpreted as changes in surface color rather than in terms of shadows or lighting.

As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view.

Edward H. Adelson