I agree that pressing movements like flat bench, incline bench, and DB bench should be at the forefront of a mass building routine for the chest, but duhast is correct here in saying that flies do have a place.
Without them, it is like only ever doing overhead pressing for the shoulders and never doing rear/side lateral raises... sure the muscles will grow, but don't expect complete development.
Flies (be it DB, cable, or pec deck) make for a great "finisher" at the end of a chest workout.
Take two twin brothers, put them on the same program, except have twin one do flat, incline, and DB bench... and twin two do the same thing, except finish it off with cable flies.
Fast forward a couple years, guess who has the fuller, better looking pectorals?