Question: I'm trying to copy an object out of an SVG file using inkscape, however when I paste it back in to a new inkscape file it's no longer the svg version, but a rasterized version.  

I'm not sure if it's because inkscape runs in XQuartz and the clipboard is behaving odly.

Answer: 
I solved this problem by *importing* another svg file into the current one.

File --> Import... --> Select "All Vectors" (or "\*.svg") in the filter selection menu.

This copies the entire content of the imported file into the current one in vector format.
This is not really a solution but a workaround I learned from [this page](https://dawes.wordpress.com/2015/10/19/inkscape-clipboard-fix-on-mac/) and it requires you to flex those muscle memory of `Command``C` and then `Command``V`.
The workaround is to use `Control``D` to duplicate instead of copy and pasting. Off course this does not work if you want to copy from one file to another or in/out of Inkscape. 

(It is a shame that Inkscape developers are ignoring all mac users. Looking forwards to see a real native implementation away from crappy X11!)