More and more these days while holding workshops, I notice fewer photographers using neutral density graduated filters. Their argument being that they can take several exposures and merge later on using software such as Lightroom. It's a valid point. Why endure the hassle of fidling with expensive resin (particularly while on a cold, windswept beach in January) when you can simply do the work in the warmth of your own home?! There's a strong case for both methods. On the one hand, there is the simplicity of shooting images at varying exposures and then merging, and also the less glass there is infront of the lens, the less prone it is to flare and filter imperfections. On the other, there are times when grads are absolutely necessary in order to achieve a desired effect. Finally, I believe there is a great deal of satisfaction to be had from using graduated filters and reviewing the 'finished article' either through the viewfinder (in the case of mirrorless) or on the camera's LCD.
So, unless you're a whizz with Photoshop and don't mind spending an age processing, not all scenes can be captured without graduated filters. Take the use of ND filters for example, where your intention is to produce a long exposure of, say, 1 minute, thereby creating strong movement in the clouds and the ocean as an almost level, textureless surface. If you're shooting at any time other than dawn or dusk, regardless if you're using a 10-stop ND, unless you use a graduated filter, the sky will simply be too bright for you to reach such a long shutter speed. You could, of course, stop the lens right down to its minimum aperture but you then run the risk of diffraction. A 2 or 3-stop graduated filter, on the other hand, will darken the sky and bring it closer to the exposure for the land and enable you to reach the all-important 30-second to 1-minute mark. There's also the issue of the foreground incorporating swaying vegetation and/or trees protruding the skyline. If you ignore grads and instead take the merge approach, you'll end up with a multiple-exposure effect of the grass/trees at different stages of 'sway'. It's horses for courses.
How do I, personally, decide when to use or otherwise? If the conditions are stable, I'm not shooting a backlit scene, and/or I'm after a long exposure, I'll use grads. If I'm not going for long exposure and/or the conditions are such that using them would be pointless such as during heavy rain or a snowstorm or if the light is dynamic, I'll bracket and merge. My advice, keep an open mind and use both.
