I'll hand-cut some odd-shaped designs when I can do that quicker than I can set it up on the cutter and wait for it to do it's thing.
Also, some square or rectangular printed projects that have ink coverage all the way around the border like a 12 x 18 rectangular sign with white letters and a red background can easily be cut using a straight-edge and cutting along the border. A full coverage banner comes to mind as that won't go through the plotter and the crop marks would only be tight against the background ink coverage anyway.
Each job is different but I'd say I hand-cut about 15% of our vinyl. Sometimes messing around with the plotter just takes more time.