If you draw a circle in inkscape, and examine its code (the <> button), you see it's actually a path (which can be a spline or polyline). But, there are a couple of "sodipodi" attributes (the ancestor of inkscape) that allow inkscape to know that it is in reality a circle. This is common to inkscape, they draw everything as paths, but they use these sodipodi attribute to "know" what type of element it is. Other applications, unaware of the sodipodi attribute, still render the SVG correctly, no matter what new features inkscape developers implement. And this allows them to implement new stuff without breaking the SVG standard.
Since we use inkscape files quite a lot, it might actually be interesting to support some of those attributes. Anyway, converting from/to SVG and DXF is something doable with reasonable guarantee, at least for basic object types. I use freecad for that quite often, actually