I've never ejaculated solely by prostate stimulation, but I do admit it adds to the orgasm of the male when it is stimulated along with normal sex. Normal males ejaculate fairly easy anyways ... possibly by nature's selection, but the stimulation of the prostate tends to make the ejaculation stronger. A talented woman, with a nice middle finger, can really help a guy have an awesome orgasm by rubbing his prostate, either internally or externally.
Prostitutes often do it simply to get the guy to cum fast so she can have another customer.