I made a John Pearse-style armrest (as Default displayed, above) for one of my 12 strings, right after seeing one for the first time. Cleverly, it attaches non-destructively with double-stick tape, but only at the very edge of the guitar. It incorporates a step design which allows it to operate as a ledge, keeping both itself and your arm away from the soundboard. It adds to the depth of the body along with adding a bit of weight to the guitar, both of which may take some getting used to. Otherwise, I think it can be a viable accessory. In some cases or for some players though, it may be a solution looking for a problem.
Your arm can certainly affect the sound of the guitar. There is no question that draping your arm across the lower bout will have a muting effect. A simple test involving laying your arm on the soundboard while plucking the strings will prove that to your own ears. However, the degree to which the sound is muffled by minimal contact as it crests over the binding is relative to the amount of actual contact (and the ability of the measuring device used to detect it). The larger the lower bout (width and/or depth) relative to the length of the player's arm, the more contact potential. Depending upon all of the pertinent factors, posture may help to minimize contact, at least up to a point.
Sweat is a definite consideration. I have seen firsthand how aesthetically damaging it can be over time, although it has no (audible) affect upon tone. That sticky, gooey muck that used to be a hard, glossy finish is undesirable, to say the least. When sweat is present, if you do not play with a long-sleeve shirt, or your guitar/arm length/posture would force contact, and you can predict sweat damage in your future, the John Pearse-style armrest may be appropriate. Now, for those of us who like to rest our chin or lay our face your face on the upper bout…