Salads suffer from non-emulsified vinaigrette sauces. Some bites will taste too vinegary, some too oily. If the cook merely stirs the oil and vinegar together with a few fork strokes, he has formed an oil-and-vinegar sauce, not an emulsion. Even if the cook does a good job of drying the leaves, their surfaces will retain their natural water content. Since oil and water repel each other, the water on the leaves' surfaces will "push away" the oil, diminishing the oil's chances of sticking to the leaves.
In contrast, the oil in an emulsified sauce will not be repelled by the water on the leaves because the oil in this sauce doesn't come in direct contact with the leaves. Rather, the oil exists as minute droplets sheltered within the vinegar. Each droplet is separated from the other droplets by the surrounding water. Think of thousands of discrete, evenly scattered beads of oil in a glass of water.
** Asian Recipes