One other argument for "diffusion" or Cold Light enlargers is that they call for a negative with a more expanded range, which also enhances separation between intermediate values, or tones in the mid ranges. The reduction in the "callier" effect (no relation!!) is most present in the highlight, or more dense portions of the neg, so separation in the mid tones is seen to be enhanced by the cold light system. I am stating this passively, because, as posted above, I am neither a scientist, and have made only one comparison, a very long range indoor architecture shot (4x5) with direct sunlight on a wall, which was the first image I ever printed with the cold light. I acheived a quality of separation and total range not possible with my condenser enlarger, at least not at the time (1980 or so). With the added benefits of less dust (and other surface defects in a negative) reproduction, lower tremperature (no neg buckling), absolute evenness of illumination across the entire frame, and no need for alternate condensor arrangements for different neg sizes, I never used the condensor system again.
Fred was not the only proponent of cold lights, Ansel Adams being one of the early ones, and plenty of others over the years.
As kobaltus states, I feel that there is a difference, for me especially in the mid to low values, a better kind of tonal separation. In my mind, I'm thinking that this may be due to the expanded range in the neg, although, it could be argued that most of the expansion from normal HQ developers is in the upper ranges, not the lower.