This is true but also helps keep gas fumes from building up after shutting it down on a hot day. My '70 Duster reeked like gas when I had the line unhooked and just hanging in my engine bay. Bought an old charcoal canister from a FABO member for a later A-body and hooked up the carb bowl vent and fuel tank vent to it with the vacuum line going to ported vacuum on the carb; now the smell is gone. I also don't have to worry about my car randomly going up in flames lol...
If you read the threads by slantsixdan he says how the original system like ours with the crankcase holding the vapors didn't work very well; it would cause an over-rich condition at idle and dilute the oil with gas over time.