Multiple products prevents shifts in weed population. 2,4-D by itself will kill dandelions. However, clovers will become dominant. I am all too familiar with what happens when a herbicide with a limited spectrum of activity is used to the exclusion of other products. Glyphosate resistance was no surprise to me. Even before that trait was engineered into cotton or soybeans, I have known about some broadleaf species resistant or less susceptible to glyphosate back in the 1990's.