If they implement the ability to click+drag multiple buildings at once -> [F] [F] [F] [F] [F] [F] -> you would want it to be the way it currently is. Which I think they are working on. If it was how you said, then they would all exit into each other
Oh, and maybe paths for separate bots/tanks/planes. So you can have your fabs go one way and your combat bots go another
Factory's can be exited which ever way you want. If the factory is facing 12 o clock and 6 o clock set a rally point to which ever side you want the units to go. If you set it after the factory begins to build there is a 50% chance the unit will exit the wrong way. Well unless you can distinguish between front and back defaults which are quite minimal.
I agree there should be an easily distinguishable roll-off direction when you place a factory. Obviously you can just change the direction after you place it though.