Well, basically because the Xonar Essence can eventually serve other purposes, like a nice headphone source, or a third rig (I have a 2nd one).
But if the Hiface is clearly superior, at 150$, it is a no-brainer.
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No WAY!
I had a Hiface and sold that crap. Essence as digital transport is near from the top I had occasion to hear (audiophilleo usb to spdif is a bit better depending of the taste).
It's my second best transport.