Who said you couldn't cut a perfect internal mitre? Not me or anyone else as far as I'm aware. If that is your preference then by all means do it. However that was not the point of the argument so don't try moving the goal posts!However you try to dress it up to suit your needs, a scribed joint is not a mitred joint. They might look the same to the untrained eye (indeed that is the whole point) but as "Troq" quite rightly points out there will never be a 45 degree angle visible on the top of a scribed joint for the simple reason that it will be a 90 degree one.