In addition to what jspec rsx said:
When you're in simulated 6th gear at 50mph, you'll have a rough time accelerating or going up a hill. Try it sometime, you either won't get the performance you want from it, or the car will override you and kick down to 4th or something.
If you don't need to accelerate, then you can put it in 6th and get better fuel economy. That's what the paddles are there for.