I remember reading that there should only be a couple of threads showing below the rocker bottom.
I thought that if the adjuster was all the way down that would put a lot more stress on the system?
Your memory and your statements are correct.
It's basically a simple lever effect being amplified, this as the adjusting stud presents greater exposure under the rotating rocker arm body increasing the distance from where it is affixed to the pivot point juncture for pushrod; and yes perhaps some valve motion is lost due to the lateral movement invoked here, but it is greater potential for part failures that generally trump other concerns.
Also, particularly in this instance, remember that proper pushrod length is established only after the rocker arm positioning in the "valvetrain geometry" equation has been concluded.
Scott.