Hrm... I think you've missed my idea. Even though the structure is laying horizontally, like an aircraft on a runway, no
horizontal motion takes place during launch. It just lifts off vertically and then rotates 90 degrees to decrease drag.
The point is that even if the rocket is free, raising it into position is expensive and time consuming. Think of the Saturn V mock-up at Huntsville. Building it was pretty simple (since it's not a real rocket), but standing it on its end for the display was a bitch, and the model didn't even need access for support (a Saturn V launch tower, crawler, and VAB). If you left it laying flat when on the ground, maintenance and support are pretty trivial. All you'd need is a step ladder. You don't have to pump fuel 300 feet vertically, you don't have to design a tower that can withstand rocket exhaust, and you don't have a tower you can crash into on the way up.