Someone may ask why do "we" aggressively look for a fuel station when we're only 1/2 empty. There is a difference between diesel and gas. Diesel is JUST not as available as gas. Many gas stations do not provide diesel. That in itself creates a challenge.
Second, I know ever state is different, but in Indiana there's 2 prices for diesel. One price is called "car diesel", for individuals. The other price is for commercial consumers (truck drivers) that get a tax exempt break. So when pulling into a truck stop pump for diesel, it's really a hassle to make sure you're paying the right amount. It means often going inside first, leaving your credit card, some such thing like this. It's just a real pain-in-the-neck if your not a commercial driver. So, conventional gas stations are much easier there because they have only 1 price... car diesel. This avoids a lot of headache.
Third, as everyone knows who drives a diesel, you NEVER want to run out of fuel. If you do, you've got to go through the process to prime your fuel filter and all of that. It's not like a gasoline engine, that you simply crank the engine and the fuel pump will pump the fuel to the engine. I've never run out of fuel, and I never want to either. Thus the reason to aggressively start looking for a fuel station at 1/2 empty. It give me a wide cushion to find that next station.
At home, this really is not an issue. I know where the fuel stations are. At home I can monitor my usage and plan my fill-ups. But when traveling in new territory we've never been to before, that's when the 1/2 rule REALLY becomes important. You just don't know don't know how far the next station will be, and if you do, that station might not have diesel. That's were applications like GasBuddy.com come in so handy.
I just wanted to explain why so many have the 1/2 rule! Hope this helps explain for all you "gas" users who don't have to deal with the "diesel" issues.