It is true that nutrients are destroyed by heat. However, different types of heating destroy food nutrients to different extents. So, if you're a raw food enthusiast who doesn't always have the time to prepare raw food meals at home, or who enjoys eating out with regular-eaters once in a while, it might be helpful to keep the following scale in mind. Microwaving and deep-frying are the most destructive to nutrients, vitamins, minerals, and enzymes. Less destructive are baking and grilling. And steaming veggies actually doesn't damage them to a great extent at all.
One of the explanations I have heard for the difference between raw and cooked food is that cookings kills the living enzymes that are in fresh uncooked fruits & vegetables. There is a book out about warming living foods without destroying the nutrients. It is called Warming Up to Living Foods by raw foods expert Elysa Markowitz. Another book I recommend about raw food and nutrition is The Raw Food Revolution Diet, by Cherie Soria, Brenda Davis & Vesanto Melina.
I was interested in your comment about steaming not destroying the nutrients. According to raw crazed people, the food can only be raw. I would like to know more about this so if you have any specific info that would be great!
A friend of mine did an experiment with the microwave just using water for tomatoes.
Tomatoes that were given only microwave water looked completely deformed.
Made me take a second thought on the "convenience" of the microwave.