It's easy to understand why you have 1 & 4 open for takeoff and until 2 & 3 are equal to 1 & 4 plus reserves. But then we leave #2 open to pressurize the manifold??? Why??? Saying "it's been that way for years" doesn't explain anything. Why #2.
I know many carriers who don't use #2. Some use #1 and some use #1 & 4.