To fully support a 3DM then you have to support a NURBS object. most rendering engines don't render the NURBS object anyway, they only render the attached render mesh. So probably the engine's authors only prefer to support mesh objects. the exception of course is Renderman which can render the Nurbs model directly (and other Renderman engines such as AIR which does have a free 3DM link). But AIR costs about $500 so i guess it's off-topic in a "free renderer" discussion...

jonah