We established recently that you can instance curves from within sketches perfectly well using instance geometry in modeling. That means outside the sketcher.
From within the sketcher you can mirror about and axis, which is a special case as dimensions can be applied to span the symmetrical elements (i.e. like a diameter or and overall dimension).
This covers just about any case you should need to create most kinds of geometry. You may disagree that it doesn't do so the way you first thought of, but it still works just fine.
Come join the PLM World NX Design SIG and find out what's coming down the pipe. There's a big project we are partnering with Siemens on for this type of functionality, as well as other "Copying" functions.
