for me the advantages are in the tools that work differently - hence my 'screwy' analogy I suppose

xara has transparency that can be built up in many stages [ by making the single object a group and applying another transparency as many times as you want within reason]
inskscape gives each color in the palette an A [alpha] attribute as well as RGB
so for each color you can adjust the opacity, completely independently of everything else in the object, because color is defined RGBA not RGB

xara has tools that inkscape does not - eg customisable [to an extent] brushes
inkscape has a 'vector flood fill' tool - strictly speaking fill bounded areas - but these areas do not need to be fully closed - a big advantage to a cartoonist

this is not to say these issues cannot be worked round so that only one program need be used - but I am not and never have been a one tool fits all kind of guy