I used to use different sets of grunge brushes for my work and I have been told more than once, I have those brushes. If your brushes aren’t brushes you made, what’s original about it? Using pre-made Photoshop brushes may get you what you want fast, but it just doesn’t compare to knowing and being able to tell people you did it all yourself. Anyone can make their work original and unique by taking a little extra time to play with some paint, so why don’t they? Not only is doing it yourself the best way to get realistic grunge, but also it shows the effort put into your work.