Besides the fuel pump cooling effect.
Another consideration is the comtanination in your tank. When you have 17 gallons of diesel all the particles and debris are "diluted" but when you have only 2 gallons in the tank everything settles to the bottom of the tank and the debris will be sucked closer to the screen. So the chances of clogging the screen and getting particles in the fuel system are greater.
Second item. When a fuel tank is near empty and the air cools overnight the moisture in the "air" in the tank will condense.This will in turn add water to your diesel fuel. Something you don't want. A full tank does not have as much "air" in it, so there is less moisture that can be condensed out.
We see this all the time in aircraft. So much that on every preflight you drain fuel from each tank to check for water in the fuel. The water will settle to the bottom of the tank and that is where the drains are. Of course we also check for contamination and the correct color to ensure you have the correct octane. You would be suprized how often water is found.
So there are a couple of reasons for keeping the tank above 1/2.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car