It seems to be that a salad is intended to be a light dish. It is usually a precursor to your heavier entree. The key is having fresh vegetables and other, similar items (such as eggs, bacon, tuna, etc.) in the salad. You want to be able to enjoy their fresh, individual tastes. So, it makes sense that any dressing applied to the salad should also be light, with some flavor of its own but not so much that it overwhelms and conceals the flavors of the salad itself. It should provide an accent, a compliment, and not take center stage.
So why is it so common to find thick, cream and rich dressings atop salads? Russian, Blue Cheese, French and more. That seems to run counter to the purpose of a salad. Those dressings would seem to conceal the flavors of the vegetables, making the dressing the most prominent taste. Plus, it makes the whole dish much heavier, and thus your entire meal heavier too. Is that really what you want? Why conceal your fresh and tasty vegetables?
Another good reason not to opt for such heavy dressings is that they are usually less healthy than vinaigrettes and other light dressings. Seems to defeat the intent of a healthy salad to add a less healthy, thick and creamy dressing. It is like eating a healthy piece of fruit for dessert, like a banana, but then adding chocolate sauce onto it.
Savor your vegetables! (Yes, that is really me talking). Don't hide them behind a thick curtain of dressing.
If you like creamy, rich dressings on your salad, please tell me why.