in my workflow i can see a real use for opacity msks as a means of quickly repostioning/reorienting a 'visibility window' that is punching through an object; pity that to actually edit the mask shape you need to go 'inside' [like clipview] which means you lose sight of the context and so the usefulness

I never went much for the non-destructive aspect of xara and could quite happily live without it... 'ctrl+z' is usaually all I need; like egg I save out asset though in my case i export rather than pasteboard if it is specific; indeed having to make bitmap copies to fix before applying trace or effect is a nuisence to me

the one area where it is useful though is erase/shapebuild which I now see as a form of opacity masking...