I would say that depends on where you are using it and to which surface you are applying it.
Say one of your plastic panels vibrates. Becuase of the car's natural vibration going down the road or from your tunes? Say you put some Dynomat behind the panel on the sheetmetal behind it. That may or may not help the rattle. Depends on if the space behind the panel was thin enough that the Dynomat touches it probably. What if you apply the Dynomat to the back of the panel instead? It may still rattle but at a different frequency? Hard to really say. Make sense?
Most often chasing down rattles involves trying to make the panel fit better around the clips or somehting like that I think. I wouldn't get the stuff just for the rattles or expect that using it would cut down on noise AND expect the rattles to go away.

 

Makes perfect sense and cleared up a lot of my confusion as to the purpose of the product.
thanks a lot!