Bands have a definite purpose. In my mind the usual purpose is rehab and to correct muscle imbalances that could be created by lifting weights. An example of this never completely achieving adduction with the humerus across mid line doing conventional benches, inclines and flies. A pulley or a band can be used to correct that hopefully will give a better range of motion to prevent injuries.
My biggest peeve about bands though is the strength curve. While useful for some movements it doesn't work for many movements. What I mean is the movement is easier in the beginning and gets progressively harder as the rubber is stretched.
