For about two bucks worth of brass(I have never seen a heater core that was not brass) I leave them in unless I have time to kill, but you had enough of the dash tore out to get at it pretty easily.
Even aluminum radiators, AC and tranny coolers are debatable as far as what is worth your time for the money you net.
I do save the anti-freeze for use in other vehicles, at $6. or more a gallon