ok, it's clear that the chainstay will get dirty and maybe the paint will chip...

but does/can it actually do any structural damage? i could car less if my paint gets chipped or whatever. i have 4 bikes and not a one has a chainstay protector. i ride about 10,000km per year - about half off-road - and have never had any problems.

anyone know of a frame failing from excessive wear from chain slap, or cracks developing in this area that a protector could prevent?

i think most of the damage is just surface and a thin protector won't do much against big damage like large hits to the area from a crash or rocks...