Mainly, it's the fact that you aren't grabbing handfuls of dirt to wave at your coil. There are bad things in holes sometimes times, razorblades, even medical syringes.
When you see the time you are saving with a handheld pinpointer, they just make sense. Targets can fly to the side of a dig, happens a lot. By the time you pick up your machine a couple times, you could have located another good signal. Add in elements such as hot or cold weather, plus any time constraint for the hunt, a pinpointer is indispensable.
A small silver dime, in a dirt clod about the same size, can wreck someone's patience level, who wants to wave dirt at a coil. jmho