Fortunately for all of us with opinions, just as in love and war, all's fair in art and painting. If you find a gizmo like Carder's tool useful, go for it! After all, the only thing that really counts is
results !
Now for the annoying opinions - (
and of course, mine are the correct ones! 
)
While looking into (or through?) a device like this may be profitable vis a vis what new perceptions of color and value may occur to the painter, the premise of using it to "match" paint to colors from the life overlooks the basic problem we encounter with color and value with all subjects each time we approach the easel. It appears to me using Carder's tool through the course of painting a picture would be cumbersome and slow in the extreme.
As color and values in nature run the gamut from looking directly into the sun to total darkness, the corresponding range on the palette from whitest white to deepest black is merely an infintisemal fraction of the natural range "in the life". It follows that painting a picture, one does not "match" color and value in the process of painting, but arranges reasonable approximations by manipulating the tiny range which paints provide to effect
illusions from bright sun to stygian blackness.
Claude Lorraine was such a proponent of viewing subject matter through a blue glass (and a few other aids for perceiving color and value) that he lent his name to the "Lorraine Glass". Primarily he used blue glass to assess relative values for landscapes.
Another useful tool in the studio for appraising value relationships is a black mirror . . . a piece of glass painted black on one side, or black plexiglas will do.
The simplest way to assess value relationships in subject matter is to compare masstones from the life with a card printed with a gray scale in ten steps from white to black, guaging approximations of value from subject to palette. Another gray scale placed under a glass palette on the taboret makes paint-mixing easy. Paint sample cards from the paint store can be used as the relative scale.
Another useful "speed trick" (especially for portrait work) is to paint a patch of neutral grey, midpoint on the value scale on your arm palette. The most daunting task facing painters is to begin working on a stark-white canvas, as even relatively light values appear excessively dark . . . hence the popularity of working into a mid-value imprimatura toned to the color temperature of the subject or contrasting with it.