Figure 4a shows the summation of signals from each of the transducer elements when the time delays are properly done. Figure 4b shows the summation when the time delays are slightly miscalibrated. First, the length of the resultant pulse is longer. This reduces the system's axial resolution. The second effect is that the additional signal strength gained from coherent summation of the signals is significantly less than in the well-aligned case.
Image quality is certainly the primary reason to select a digital beamformer.
Color flow imaging involves sending out a burst of sound to a specific position in the tissue and waiting for the echo to return. Most of the time spent in this process is consumed waiting for the sound to travel out and back. To make matters worse, the process is repeated for every pixel in the region of interest. This means that the system spends a considerable amount of time waiting. This is time that cannot be reduced with a faster computer or better designs.