Mission Essential (in that if you don't have one you'll fail)? No.
Mission Enhancing, definitely. It will obviously help you speed up a learning-training cycle.
Will you fail a course of fire (or miss a trophy animal, or worse yet not connect on a bad guy when you're providing overwatch protection) if the gadget fails for whatever reason (it's out of batteries, its lenses are covered in mud or snow, you've lost it, or you simply can't get it out of its pouch and turned on)?
Exceptional field shooters will use the entire range of "Cheats" (electronics like lasers and Kestrels) and anything else (Known Distance ranges, standard-size steel IPSC or E-type silhouettes, smoke, wind-, and range flags) to train to the point they can't mess up a call so badly they can't score a hit.