The round tube vent in the choke tower will, at high air velocities (W.O.T.), create a low pressure area in the float bowl. (air rushing over the tube creates low pressure in the tube. That's why it's cut at an angle, to reduce this effect) Adding a secondary bowl vent eliminates this possibility and insures that atmospheric pressure acts on the fuel and not something less than that. Evidently, this was only a problem on higher performance engines capable of high air intake velocity. That's probably why you don't see it on all carburetors.
If you want to witness this effect in action take a look at a Quadrajet on a dyno with the secondaries wide open and you can actually watch fuel pulled out of the float bowl, up the vent tube, and in to the secondary throttle bores. I used to make air dams that disturbed the air flow over the vent tube to eliminate this. It was wild to see it happen.
I know I'm a newbie on here and I hope I'm not boring people to death with my technical ramblings. This just happens to be the only automotive forum I've discovered that has a lot of intelligent people.