The issue here is does the parameter the gage "measures" affect the quality of the part. That is to say, you have to look at the tolerances and determined the effects of the parameter being measured.
I agree with Hershal as far as his basic statement goes - If it is not telling you something of value, it's not needed.
BUT (You were ready for this, right?) - A "Black Box" is harder to control than one with "relative" indicators. For example, probably everyone here drives some sort of vehicle with a gage telling them how much fuel is in the tank (or battery). In my car I also have an oil pressure gage, a gage which tells me if the alternator is charging the battery or not, a light that pops on when one of the tires is loosing pressure, and - Well, a *lot* of "information" - Even approximately how many miles I can drive before I have to stop for gas based upon how much fuel "it" thinks is in the tank taking into consideration it's analysis of the MPG the car has been getting over the previous 25 miles.
To some degree a gage may be *relatively* "critical", such as an oil pressure or engine temperature gage or light. None of these are calibrated, much less precise. But - They help me (and you) to understand some of the somewhat critical vehicle/engine operating parameters. If the engine oil pressure drops to zero and/or a light or alarm comes on you know to stop right away.
How "precise" is this? How "critical" is it? Is it useful? Should it be calibrated?
This is no different from a lot of process equipment, including injection molding. There are often "relative" gages or measurement equipment which gives a person some idea what a piece of equipment is doing in certain aspects.
In years past I have gone through this with auditors many times. The answer is always simple: There has to be someone in the company
(including everyone who operates that piece of equipment) who knows about, and can explain, every measurement device on a machine/process equipment and why it is there. They also have to be knowledgeable about the process.
This determination *should* have been made at the stage where the equipment was contracted for. That is the time when decisions about the Critical Characteristics of that process *should* have been identified and any critical process parameter measurement equipment defined. The Critical Process Parameters defined at that stage should be inputs for the Process
FMEA and Control Plan for the process.
The person who deals with the auditor should be able to explain why every gage is on that piece of equipment and why each one is,
or is not, *critical* to the process.
I have seen a couple cases where auditors did a calibration writeup because every measurement device in a company wasn't calibrated. Both were challenged and the auditor lost both times. There is no standard that I am aware of that says that every measurement device in a company, no matter what it's purpose, has to calibrated.
If a measurement device is *critical* to the process (and thus the quality of the product), or is used to measure the "quality" of the product, then it must be calibrated. Every company has some measurement equipment marked, and used as, "For Reference Only".