The problem is for users, like me, that are more use to a photo environment than a designer one. The moment i've read about the artbooks my mind exploded and even the export persona started to make much more sense. I guess this is solved for me but there will be other people who could ask for this option and @MEB could point to this concept :). There are tutorials, but we need a pointing finger. Guys, if you need to crop a doc, subfold your piece under an artboard, which is even selectable in the export persona with the hand selector, nice and easy!
Maybe the hand selector could be the primary mode as sometimes I go to export with nothing selected and the first tool I click is the slice (crop) tool -habits are hard to beat- or at least make the slice tool in the export persona snap to the first layer it touches (which for noobs like me is going to be the border of the doc, again, old bad habits). It's horrible to be a noob and blame a more clever software than I am. :D