No... you said it perfectly. What you don't understand is what is shown in this diagram. Getting power to that end of the layout entails crossing over a bridge and coming at it from the other end. If I use a contact system on the door opening section (the left side on this drawing), I could greatly reduce my wiring runs to that end of the layout. Then I could use your idea only coming at it from the front, not the back.
The wire is shown in red. Remember, the entire middle is open so wires can't cross there.
