The thermostat is important for 2 reasons. First now that fuel is computer controlled it is important that the engine reach designed operating temp asap. Till it does, the temp sensor sends a signal to tell the computer to run a rich fuel mixture.

The second reason has always existed. A car without a thermostat is always prone to overheating. The reason for this is that the water moves too quickly thru the radiator. It will move so fast that it doesnt have time to properly exchange heat to the air. I have seen many people remove thermostats thinking the engine will run cooler and they accomplish the reverse. The thermostat will adjust the flow of water thru the radiator and insure that it exchanges the right amount of heat.