Worst thing is... if they idle their cars while waiting, it'll end up costing them more money in gas than if they filled up next door. A car idling wastes about 1,2 to 1,6 l/hour IIRC, so if they idle for 15-20 minutes, they'll waste 0,3 to 0,5 liters, times 90,6 cents, that's around 27 to 45 cents of gas wasted waiting.
I personally fill up basically always at the same stations because they're on my way to and from work and I don't have to waste time going around a bit, even if the price of gas is 1 or 2 cents more per liter.