In the early 1960s, liquid soap was always used as a surfactant by spray companies and local governments who sprayed, such as counties or towns.  I have been using a generic brand for the last two years and have seen a noticeable improvement on killing larger weeds than using nothing.  Chemical supplier at one location told me it doesn't work.  Chemical supplier at another location said I should just keep the secret to myself so he could still sell the ammonium sulfate. 

As always, if something works and is lower cost, it only makes sense to use it.