Model glasses are an approximation, although usually a good approximation in the visible range.  Outside this range, however, such as in the ultraviolet or infrared, the model glass is not accurate and should not be used.

It is true that even though the model glass is a good approximation in the visible spectrum, it should NOT be used as a replacement for other methods used to create glasses in ZEMAX if the required data is available to you.  In other words, if adequate dispersion data is given to you for a material, use the alternate glass modeling methods in ZEMAX becuase they are more accurate.  However, if the information available for the glass you wish to create is limited to the three parameters, the model glass in ZEMAX is relatively accurate and reliable in the visible spectrum. 

In monochromatic designs, the model glass may be used to represent the index of refraction for the design wavelength very easily.  In this special case, the Abbe number and change in partial dispersion terms should be set to zero.

For high quality optical systems, the model glass might not be an adequate representation of the dispersion of the desired glass, and should be used with great care.