the mapping in arena seems to be a utility ( ie: something you setup and leave ) whereas mapio has uses as an effect ( something you can change around whilst in the mix - this is where the presets rule! ) i have a library of presets or "looks" designed for both 4:3 and 16:9 screens which i can dip into at any point.
the attached pic shows layer 1 ( orriginaly just 1 orange circle ) has one mapio instance applied and layer 2 ( orriginaly 1 cross ) has a different instance applied - but the key is that the mapping effect is restricted to that layer and the mapping for each layer can be switched on the fly. as the mapping in arena takes a slice of the comp as a whole it doesnt work like that. the only way is to restrict your layer to a section of the screen which in turn reduces the res of each image and means you cant use blend modes.
hopefully this shows how mapping can be an effect as well as a utility.
*edited to change the pic as the last one was huuuuuuuuge! *