I think it's odd that a reputable engine builder would suggest leaving it out.
Besides taking longer for the engine to properly warm up without one (and there is no up side to that), you could be crippling your cooling system.
The thermostat also works as a coolant flow regulator. Without one, coolant will often flow through the engine and/or radiator to fast to properly transfer heat away.
Depending on how effective your cooling system is two things can happen.
Often the coolant flows through the radiator too fast for it to remove the heat built up in it, and you will get higher coolant temperatures.
Even more of a problem, if you have a very efficient radiator & fan setup, it will take the heat pout of the coolant, and everything will look OK on the coolant temp guage, but coolant could be flowing through the motor too quickly for it to transfer heat away from the engine properly. The coolant temp guage looks nice and low, but the head temperatures are soaring and detonation becomes a problem (or you simply can't run as much ignition advance as you should be able to).
If an engine has overheating issues, It needs a larger or more efficient radiator or better fans. Removing the thermostat is a band aid, and not a good one.
Often times those people are running too strong a coolant mix (too much anti freeze). Anti freeze does just that. It prevents freezing. It also adds some anti corrosives, but it is less effective as a coolant that water.
A cooling system running 70% water is going to run measurably cooler than one with 70%
antifreeze will.
If your in a time of year where freezing is never going to become an issue, and your having high coolant temp problems, try running straight (distilled) water and see if the problem goes away. You should still use at least 15%
antifreeze for the anti corrosives.
Try running a wetting agent. They do work to lower coolant temperatures whether your using a 50/50 mix or straight water.
The way they work is, when water does start to boil, the steam bubbles that form will be much smaller than they would be otherwise, which leaves more coolant in contact with the engine and radiator surfaces.