Grip strength can cover a lot of ground: crushing, holding, pinching, twisting, bending, tearing, etc.
Unless you're a climber, arm wrestler, or something similar, I don't think most people need to spend much much time on all aspects of grip. I'm mean, some grip tricks are kind of neat, like rolling frying pans, but at the end of the day it's still just a grip trick.
I just feel if hands are not a primary goal, training time can be better spent. At the same time, if someone just wants to have strong-ass hands, who am I to judge their goal?
I think most gym-goers will never need to more than some basic grip work. Farmers, barbell holds, and thick bar work will cover most people's needs. Throw in some gripper work and some wrist curls and a person can develop some solid hand strength.
Now, to have world class, freaky hand strength, you need to specialize your training. If you want to get good at bending nails and tearing decks of cards, you have to practice specifically doing those things. In my experience, I haven't seen a lot of carryover between the different aspects of strength.
For example, I can double overhand pull 405 and hold over 600 with a mixed grip, but I can't close the COC #2.