Is there anything important to know about exhaust temperature for diesel engines? Why is it taken?

High EGT's will destroy the engine and/or turbocharger. Also high EGT's may mean that you're running too rich.

No, high EGT's usually mean you are running too lean.

Not in diesels... excessive fuel injection (basically adding more fuel than air) makes high EGTs.

When tuning high performance diesels, EGT is a direct indicator of danger. High EGTs will cook a turbo. With diesels (since you simply use turbos to add air and things like timing, pressure, and duration to add fuel) the addition of power is more of a bluntly applied increase. With gas engines there are a thousand things to worry about; intake manifold design, cam timing events, exhaust tuning, etc. With diesels you worry about three things; adding more air, adding more fuel, and adding more exhaust to get rid of the spent charge. For that reason, higher EGTs are an indicator that you have exceeded fuel input based on air input and/or the exhaust's ability to get rid of it.

just to clarify what curtis said, the reason an excess of fuel will give higher egt's is that the extra fuel exits the combustion chamber and keeps burning in the exhaust instead of having some of the heat energy transfered into motion or transfered into the surrounding metals.

The highest exhaust gas temperatures occur at slightly above lambda 1. Gasoline engines run with a lambda of one or less, diesels run with a lambda of more than one. This means that for most gasoline engines a leaner mixture will increase EGT while it on a diesel will decrease EGT.

So why is this the case? Well, lambda 1 is the most fuel that can be burned. The more fuel that is burned with a certain amount of air, the greater the temperature will be due to the higher heat release. With a higher combustion temperature the exhaust gas temperature will increase. If the fuel is richer than lambda 1 additional fuel will supply some cooling.
EGT is aslo dependant on expansion, if there is a greater expansion the gas will be cooler. This means that if most of the fuel is burned at TDC instead of later this will increase the expansion, efficiency goes up and the EGT goes down. In a gasoline engine, running a lean mixture usually slows down combustion velocity.
Some of the heat is also due to "afterburning", during combustion a small amount of fuel will remain unburned, this will slowly add heat after the combustion.

With a diesel you will have most of the problems that you have with gasoline engines. Well, the exception is engine knock. But instead you have big difficulties with combustion duration which tend to be longer at higher speeds, that is not so much the case with gasoline engines.

is it beneficial to put a Egt gauge in a non turbo diesel?

is it beneficial to put a Egt gauge in a non turbo diesel?

Can't hurt. They can still save you from some horrible damage down the road.

I guess it can be but chances are that you wont ever need it on a non-turbo diesel because they don't develop very high egt's. also, there isn't a turbocharger to worry about so they can go slightly higher without much damage, you only really need to worry about the valves burning.