It does have an effect when an entity is imported into WED. I've done it. Try importing an entity with 60,000 faces and 30,000 vertices into WED without an action. Then add an action to it with nothing more than turning the dynamic flag off and the frame rate increases by 25%. I've tested it so I know. This same thing happens no matter how many polygons I use and the effect diminishes with low-poly models like 50 polygons.

Edit: forgot to mention that ent_created entities don't have this.