A great quote I recently heard from well respected audiophile mastering engineer Stephen Marsh, is that "Mastering is the completion of the process of mixing." He views them as part of the same process and I really think that is true.
I agree, Paul. Particularly in genres such as ours where the studio becomes an extension or at least a part of composition itself, mastering is really a continuation of the process of making music. Ceding that function to an algorithm does not appeal to me. In using an experienced mastering engineer, I think it's also important for the musician to be an active participant in that process; giving feedback helps to ensure that the results are an extension of your vision.