Full Or Not Full That Is The Question

There is a joke about the car fuel indicator F is mean Finish and the E is Enough. Of coz nobody is going to be that stupid anyway. But you ever heard about that people told you do not always wait until the fuel light on then you went to refill the fuel; because if you do that the fuel you have in the tank is gonna be dirty and damage your car engine. I myself don't believe this fact, Coz as far as I know there is a fuel filter  to filter all the dirts before the fuel pump sending the fuel to your engine. 
I used to it to buy half tank fuel for my car one... the reason I do this is because I believe half tank is always lighten then full tank. So the car can lighter I personally believe is going to reduce the car fuel consumption. Don't you think so? And also I can easily monitor how much fuel I had used in a month too.