I think a bevel is indeed made primarily for comfort, although some builders (e.g. Taylor) say it makes you automatically hold the guitar differently so as not to "infringe upon the soundboard's vibration" as you stated it.
Having said this, I do not agree that the best way to play a guitar is to do it without "infringing upon the soundboard's vibration" - which is the usual Marketing story for bevels and armrests. Listen to the clip below, in my signature. Most of the time I used my entire arm and palm with a lot of force to create "the ultimate arm + palm mute". I basically blocked most of the soundboards natural vibrations to make the guitar sound sharp and wild. Even during the more 'relaxed' interlude halfway, I put my pinky on the top and pressed really hard, with modulation, to shape the tone. Only during the intro did I use a 'freely floating' soundboard.
So armrests do not solve the challenge to play guitar...
