It depends on what you mean by “mash hydrometer”. I would think all of use have a triple scale hydrometer and use it often. It tells you where you are starting out and when you’re done, at a minimum.
If you are talking about a hydrometer used to measure alcohol from the mash in a distillery, you’re on the wrong site. An alcohol hydrometer would not be of any use to beer brewers since they generally only go down to ~19% alcohol. Even my highest octane beer hasn’t ever been close to that.
A mash hydrometer in brewing is used to measure the SG of the mash at various times throughout the mash to measure the progress of the mash. They are generally calibrated at mash temperatures.
Maybe if you are using homegrown malt or undermodified malt … can’;t see any reason to use one with how highly modified the malt we have today. I’ve never used one and I commercially brewed for 10 years and haven’t known any who do either.
I could see the benefit with the rise of craft maltsters. I’ve brewed with traditionally made Sainhuss malt and it would’ve been nice to not have to chill my sample down so much.