Most issues with low engine temps deal with the oil
temperature remaining low and not circulating properly. If the oil
is thick (cold) it doesn't circulate and lubricate as good as at the proper engine temperature. Also engines are built with a heated tolerance meaning that cold, the gap between the cylinder and piston may be .005" where hot the gap closes to lets say .001". That may not sound like much but in the world of mechanical things it can cause all kind of problems. It's important to maintain the manufactures' parameters. Also figure it this way, do you think the manufacturer would add a part that wasn't necessary?
When dealing with overheating issues it is often missed that the block has become "calcified" or (non tech term) "crudded up". This could be rust or a number of other "cruddy" items that restrict or inhibit the heat transfer to the cooling
fluid. Lets say you got a 1/4" water passage
in a block that has been reduced to 1/8" do to rust. First off you have reduced the water flow by 1/2 which could require twice the water flow to do the same job. Second you're trying to transfer the heat from the engine to the water and now have to go through an additional layer of crap. This could retain the heat in the block well above the operating temperature at the heat source since you've added another layer the heat has to transfer through. Old Adobe homes would absorb the sun's heat all day and keep the interior
cool. At night when the temps dropped outside the heat would continue to transfer into the house warming the interior
at night. Once the proper heat transfer coefficient was determined you could figure the thickness of the wall and maintain the temperature inside for 24 hours. With an engine the same thing happens, the rate of transfer is slower and the heat side of the block gets a lot hotter causing additional ware.