ND Grad Filters are just another tool in the bag for making the photograph as close as you saw it. As others have said, ND Grads are good to bring the DR within the capabilities of the camera when the scene allows you to. I have the Lee system, but there are times where I will favour a 2 shot bracketed picture - typically when the part of the scene I wish to control is too irregular in shape.
Sometimes when I have used them, I will still correct the dark banding that you can often have on the scene as a result of the Grad overlapping with other elements of the scene. I will also use a Grad in LR to either lighten or darken a scene further (Lee grads only work in stop increments, sometimes I want finer control)
Bracketing, as people have pointed out, is more difficult with moving elements in scenes, but i find most of the time I'm either blending sky or water with the land, and therefore the moving elements (grass, water, trees), are limited to only one part of the scene and not impacted by the blend.
Bear in mind also, that if you can avoid a filter, even a Lee, that you will get sharper pictures so always check before you reach into your bag. How much does it blur? Put your camera on live view, 10x zoom, and put the filter in front and then away from a scene and see. It's not massive, and it can easily be corrected in PP. But if you don't need it, don't use it :-)
Your Lee circ pola will also be a good investment once you need to use it across a few lenses. And of course there are things a Circ Pola can do which you can't do digitally.
Finally, the other thing I like about the Lee system which I've used on occasion is their 2 rotation filter holder which allows you to independently rotate 2 filters. So you could use one Grad for the Sky and perhaps a 2nd for some water.
Like I said, filters - both real and digital are just tools in your bag to help create the scene you envisaged. Happy photography...
Oh ps. if you have not ordered them, get the £3 white plastic caps. I leave the adapters on all the time and therefore these are great protection...