I've been playing around with the Fluid Grid and after doing so and also watching a couple of tutorial videos and searching the forum I have been unable to determine the usefulness of creating a "Fluid Grid Layout Div Tag" with the "Start new row" box UN-checked. Since the only thing it seems to do is disable the ability to move it up next to another column, why would this ever be used? I created two of them, and made them 2 columns wide and since they are not cleared and are floated left, I thought they would float side by side regardless of the device, but they don't. I don't understand the point of them.
Have a look here http://viljamis.com/blog/2012/responsive-workflow/
As far as the fluid designs feature in DW, regard these as as a starter for newbies that want to create web sites using responsive designs. As an advanced developer/designer, you would not use the feature.