The reason I have the opinion I do on this issue is because I’ve had to inspect and adjust as necessary the valve lash on many foreign cars, the primary ones that use mechanical lifters. Nissan, Subaru and Honda being the primary ones for the screw and nut method.
At 2 of the dealers I worked for and in my own shop I did all of my own cylinder head valve work. There’s a lot to this issue so I won’t go into it all except for a few brief points.
Assume an intake spec is supposed to be .008 and it’s actually at .013. It will sound normal but it’s goint to cause the valve train to take more of a beating. That’s also going to cause the valve to not open as much. A measly .005 may not sound like much but with use that’s a considerable amount of air/fuel being obstructed.
If an exhaust valve is tight at .002 there won’t be a problem, MAYBE. One overheating episode or a little more valve face dishing and that valve won’t last long.
Regarding the loose valves and mushroomed stems I’ve often had to use a small file to remove the burrs from the stem (caused by excessive lash) rather than score a guide.
Inspection of other exhaust valve faces and seats during a valve job for say 1 burned exhaust valve have revealed microscopic channels in the face or seat due to tight lash.
With the naked eye it may appear fine but under a magnifying glass it can be seen.
Eventually that microscopic channel is going to look similar to an aerial view of a river bed.
In the case of some of the older Nissan they would even throw a rocker arm off if the valve was too loose and the engine was revved high.
It just seems to me that regular inspection and adjustment of the lash as needed optimizes engine performance and eliminates the risk of serious and expensive cylinder head damage.
(The poster boy for this would be the guy who apparently adjusted all of the valves on his new Subaru to 0 lash and ended up with a trashed vehicle that only had 7k miles on it. In this case both heads (all of the exhaust valves and seats) were trashed so badly even the cylinder heads could not be repaired.)
Even Harley Davidson started using hydraulics in 1949.