There have been a bunch of articles going around recently extolling the virtues of mulching in and letting your leaves decay into your lawn. While this is sound advice as far as adding organic matter and carbon, it is important to remember our friends the soil microbes. As composters know well, soil microbes mainly need carbon and nitrogen, plus a little water to survive. Dry leaves are going to be mostly carbon. Green lawn clippings are high in nitrogen. If you leave your clippings they will almost balance an average amount of leaves.
The interesting thing to remember is that the microbes are highly adapted to working in extreme conditions. If there is too much nitrogen and a lack of carbon they will vent the excess nitrogen as ammonia gas. On the other hand, if there is too much carbon and not enough nitrogen, the microbes will pull it from the soil essentially robbing it from your lawn or crops and leaving them deficient (yellowish).
The practical point of this story is that yes, leaves add important organic material like carbon, but you need to add organic nitrogen every couple of years. Other than grass clippings, good sources of organic nitrogen are feather meal (my favorite), bone meal, chicken manure, and soy. It doesn’t take much and your lawn will look great, be healthy, and use less water.