The single-tank operation on old 172s was a result of some engine power loss events that Cessna blamed on vapor lock in the fuel lines under certain conditions of altitude, temperature and humidity. What humidity does I have no idea. Anyhow, they had two service bulletins on in the late '60s and a service kit to fix it. That kit involved installing tees in the vent crossover tube above the headliner behind the front spar carrythough and a couple more in the fuel lines above the doorsills where the fuel moves out of the tanks and back and down behind the aft doorposts. The idea was to let any vapor bubbles that formed to escape into the vent line.
Cessna tested the original system using clear plastic lines and a movie camera to try to spot how the vapor formed and what it did to the fuel flow. They weren't able to get vapors to form on their own, so they had to introduce some air and see what it did. They designed the fix based on that. They never were able to confirm that the power loss complaints were due to vapor lock in that area, but that was the most likely spot.
From their info in the service bulletins, I inferred that vapor might form there if hot and high enough, since the head pressure is at a minimum there. A bubble in the line might want to move upward at the same rate as the fuel is travelling downward, so it is, in effect, a partial blockage in the line. That, I think, is why it was OK in climb (lots of fuel flow at high power that carries the air to the carb where it is vented through the bowl vent) or in single-tank operation in cruise (maintains a good rate of fuel flow in the line, twice that it would be if on Both). Cessna installed the modified system in 172s from 1969 and on, IIRC.
I also use it to balance the load as mine likes to drop a wing slightly depending on configuration. I've never thought about using it to prevent fuel crossfeed on the ground. One night the FBO that fueled me up refused to put me in a hangar with an approaching storm because fuel was dripping from the vent.
