the purpose of your thermostat is to allow proper heating/cooling of your engine. the thermostat must operate under direct liquid contact. when the engine is cold, the coolant is also cold, causing the t-stat to stay closed. while the t-stat is closed, coolant only circulates within the block. as combustion keeps occuring, heat is generated within the block. the coolant absorbs the heat from the cylinder walls, head, etc and starts to get warm. once the coolant gets to a certain temperature, the t-stat starts to open up allowing circulation to the radiator. if you remove your t-stat, your car will run richer during warm-up causing premature failure of certain things like your cat converter, spark plugs, carbon buildup behind valves.
bodysheild- when you friend says "the leading causes of KA's dying due to them not opening up", that just means that theyre stuck closed..replace the thermostat like your supposed to when its not working properly and it will fix that problem. id suggest putting the t-stat in, either way, it doesnt take that long to remove it anyway.
kevgsx99-you can tell whether your car is overheating due to a leak in the head/block by checking your coolant level. the reason why your car will not overheat when the t-stat is out is becuase the coolant is constantly being cooled in the radiator. if all the other stuff like yoru radiator, coolant hoses and watnot are good and your sure that its somewhere along the line of your t-stat then your overheating problem could simply be because there was air trapped between the coolant and t-stat. the t-stat will work properly only when it is in DIRECT contact with the liquid(in this case, the coolant). also you may have a stuck thermostat in which case, you can jsut replace it and your overheating problme would go away.