Who wouldn't love to have a quiet air-cooled engine!

Factory service recommendations take into account the durability of materials and the range of conditions they will be subjected to.
An air-cooled engine can be a demanding environment.
No doubt the factory has considered the range of temperatures and use (i.e., high R.P.M.s) that the driver might subject the engine to.
Heat combined with the linear acceleration of mass does very interesting things to metal.
At various speeds and temperatures, that valve is returning home to its seat.
Your valve train is in a constant state of flux between "worked hard" and "tortured".

If I can determine the amount of service life I need to get out of my engine, and then adjust my valves based on my knowledge of the strength of the metals, the fuel I'm using, the true temperature range of operation, and the amount of tach I'll use, then
I can feel comfy with custom settings.
It's part of racing.

If you like your mechanic and his rebuilds get good service life for the way you drive your car (he knows how you drive your car, right?) then stick with him.

Cheers,
Dave