Okay i think you are looking at this a little wrong, yes you want to keep your model from being overly heavy in polygon count but you also need to have enough polys to get the shape you are after.

The smooth mesh preview allows you to preview what a mesh roughly would look like if you applied a smooth node to your geo so this is useful because it allows you to keep your geometry as low poly as possible because you can see when you have enough polygons to achieve your desired shape when smoothed.
An example might be organic modeling, you can model say a hand low poly and it will be quite boxy (for want of a better word ) but when smoothed it could be more like a hand with rounded fingers ect.
Mental Ray is capable of rendering a smooth mesh preview also so if you want you don't even have to smooth the final mesh.

As a final note very low polygon modeling is not so important these day as computers and graphics cards progress higher polygon counts become possible. Game modeling is where you need to be more careful since in a game engine things get rendered in real time and the more polygons in a scene the harder that becomes.