A thermostat has three functions. It regulates hot water going to the radiator, so you would think that removing it will lower temps...but it also regulates flow through the bypass passage, so removing it means that at least some water circulates continuosly through the hot head, without entering the radiator. Finally, it provides a restriction which increases local pressure in the head, increasing boiling point.

Removing the thermostat can have an unexpected effect, depending on which factor dominates. Generally, Mercs wont overheat, but a GM v6 can boilover in minutes without a thermostat.