I actually have valid arguments for why I do this :
1. Riding with a half full gas tank makes your car lighter and therefore you get a better mile per gallon ratio, and you end up saving money.
2. Where I'm from, in Montreal, we pay gas by the litre. Sometimes, the price of the litre can move up or down 10 cents overnight. By not filling it up all the way, if I put in 20 L at 110 cents on a certain day, if the day after it drops down to 100 cents / L , then I can fill up the rest of the car at that more economic price.