That may be true; but we're talking skimmer action here. Velocity, not flow.
The skimmer weir limits the flow coming in to maybe the top 1/2" of water. If the cross section area of the skimmer throat is 3"X12", that's .25 sq ft. Let's say the skimmer is pulling a measly 10 gallons per minute. That's 1.3 cubic feet per minute. That means the flow coming through the skimmer throat will have a velocity of 5.2 ft/minute or .087 feet per second to maintain the water level. Now reduce that opening to 1/2"X 12". That's 1/6 the area so to maintain the water level - and water will seek its own level - that water is moving six times as fast or 5.2 feet per second. That's a pretty good clip - 3.5 miles per hour - enough to overcome slight breezes and currents and pull water into the opening. Unless you have a return jet pointing straight at the skimmer throat, any surface debris being shoved along by the return flow will just go right on past the skimmer otherwise.
Look at it from an economics standpoint; if the weir served no good purpose, don't you think they'd leave them out at the factory and charge the same?