Unlike many instructions on how to use ND-grad filters, I mostly handhold mine. The main reason for this is speed, as mentioned by both Moose Peterson and Art Wolfe as their reason. The other reason is to free myself from the cumbersome filter holder. Light is often changing fast, and to be tied down with the use of a filter holder can result in losing the shot. Also to be able to not need to first dismount the filter holder from one lens, and then attach a filter adapter to a new lens. This saves time if you tend to change lenses more often. I think here in the line that you use more primes than a zoom lens.
A further reason is also to minimise the chance of vignetting caused by the filter holder. Especially with the use of “fisheye” wide angle lenses. Not referring here to the typical fisheye in the angle of view but to an optical field of view normally associated with fish-eye on the full format camera but to wide angle lenses used on crop-sensor cameras. These lenses are more prone to vignette with the use of filters in front of the lens. I’ve used both Cokin’s P-series filter holder and Formatt Hitech 100 series filter holder with the wide-angle lens-adapter, and both systems have produced vignetting with a polarizer (low frame version) attached. This is where handholding really helps in eliminating that problem.