Let me start off getting right to the point; sweating does not equal weight loss! It amazes me that in this day of fast technology and lightening speed access to information, some people are still in the dark when it comes to theories about exercise. I recently saw a guy running up the street (a bit of an incline) at about 1 pm on a hot day. He was in rubber type pants, had multiple sweatshirts on, a hat, a hood, a backpack full of something heavy, and hand weights! I should have stuck around to call the ambulance I am sure he was going to need. Maybe there was a reason for this method of training, though it would take a lot to convince me of it. I bet he is just looking for a ‘get thin quick’ scheme. Sure he lost weight that day. Sure he was sweating a lot. Sure he got a good workout. The problem is that upon drinking water he will gain the weight back. He will likely get injured and therefore not be able to train anymore, or will get burned out because his training is too tough. This will leave him at square one most likely inactive and still out of shape.
Sweat is the body's natural cooling mechanism. More sweat does not equal more calories burned. An out of shape person may sweat when exercising, but scientist have found that a person who is in shape sweats sooner since their body is more efficient at cooling down. A body that is cool has greater ability to do more work, and therefore can exercise longer. Instead of wearing rubber suits, people wanting to lose weight should allow the body to cool itself so that they can exercise for longer periods of time, or they can increase the intensity of their exercise. Working out harder or longer (not working out while getting hotter) will indeed burn calories. The weight loss from water will be quickly replaced when water is consumed or foods are eaten. There is no need, nor scientific reason, to induce extra sweating. I will allow you to make the claim that it detoxifies the skin, but how many people actually wear multiple layers of clothes and workout to simply have better skin?
Excessive sweating without replacing of the fluids and minerals will likely result in cramping, nausea, and can even be life threatening. Frequent exercisers, and most endurance athletes, will notice a change in the saltiness of their sweat. Another adaptive mechanism of the body is to try to keep minerals (sodium, potassium, etc) in the body instead of letting them 'escape' onto the skin. The result is sweat that is less salty, as evident when it gets in your mouth, your eyes, and even the way it dries on your skin/clothes.
There are ultra-marathoners who run from Death Valley to Mt. Whitney Portal. This is from the lowest, hottest place in the US, to the highest place in the contiguous US. The temperature easily spikes above 115 degrees, and the distance is 135 miles. A race like this may require some heat training, but unless you are planning on toeing the line at this race, please stay cool and comfortable. And thanks for not dripping excessively next to me on the elliptical trainer.