First and foremost, what is the dust and scratches filter? Well, its a filter that can smooth your image to remove dust and scratches from your photos, caused by (usually) a dirty or abused scanner or a damaged photograph or drawing. Lets look at an example. See, here you can see some simulated water damage added directly to the image, as sometimes the damage is on the image itself, meaning that no matter how much you clean your scanner, you're just going to have to work around it. (Photo Credit: Amber Neely)
Using the dust and scratches filter, however, will help blur dust, dirt, scratches, and damage away. However, it can blur the image far too much, resulting in a result that was much worse than your starting scenario. For example, you can take your image from that to this:
And at that point, you were probably better off leaving the dust or scratch on your photograph. So how do you work around this?