In addition to creating an even light source, there are other considerations that are even more important, in my opinion.
Originally Posted by Flotsam
The characteristics that seem to be of importance are the design and construction of the condensors themselves. Since these are in effect an additional optical system that affects the light quality prior to the light passing through the camera negative, one can see that the quality of this optical system is of importance. Involved with the design of these optical componants are such matters as the type of glass used, the optimal radius of the ground surfaces for the film format size, and the spacing of these componants.
The condensors in a well designed system are responsible for focusing the light at the nodal point of the enlarging lens. This does not occur in a diffusion light source and may not occur precisely in a condensor system that is not properly designed.
The cost of these condensors, as in most things optical, are expensive. I believe that I remember a figure of $25,000 on the condensor set for a Devere enlarger if one were to buy them today.