A word or two about:
Bill’s Forage Files: Fertilizing Hay Crops, Part 2: The Sources of Nutrients in the Soil

All nutrients reach the plant in ionic form. An ion is a charged particle, anions have a negative charge, and cations have a positive charge. These ions reach the plant in the soil solution-the soil water mixed with all the nutrients and many other compounds.

A very important source for plant nutrients are the clay particles and organic matter in the soil mixture. They are constantly binding and releasing the nutrient ions. Most of the nutrients exchanged are in the cation (+) form because the clay and the organic matter have more negatively charged (-) sites than positively charged sites. The amount of positive charges that a soil can exchange is called the Cation Exchange Capacity (CEC). This is one of the reasons why crops grown on clays and peat soils are often higher yielding than those grown on sands and loams. Figure 1 below shows the different CEC levels for different soil types. These levels are almost unchangeable, except the organic matter percentage (Some editing done here at the farm)

This is a very good explanation of the effect of and significant's of CEC.
Now my contribution, the above explains why I feel a measured CEC value is well worth the cost. A Measured CEC can be considered a long term investment, something that can be left to your descendents.
A computed CEC is a rough WAG of the true CEC value, using indicators that are dependent upon fertilizer inputs. A CEC value has every thing to do with the amount of clay and the type of clay that makes up each individual soil.