Think again.
If you wear form fitting dresses (even if they have a built in "under dress"...Erica B. tell us what that's called cuz you KNOW you knoW! LOL!) then you need to wear something to clean up the lines of the dress from underneath it. Sure...you might have the tightest body in the world but that's not the point. The point is to have an elegant line and not have folks seeing imprints of your undergarments which is distracting as hell.
This is the dress I purchased my Spanx for. Here I am first WITHOUT the Spanx: