Well, its already been said that NAD+ accepts a proton. An old simplistic definition of oxidizer is a proton acceptor -- a reducing agent give a hydrogen, oxidizer accepts it, that sort of thing. If the substrate is one molecule, and the enzyme is a catalyst and not changed, and the substrate needs to be oxidized or reduced, then NAD+/NADH are needed.
Of course, if the enzyme works on two substances, oxidizing one and reducing another, it wouldn't need NAD+/NADH. Hmmm ... I guess that just makes NAD+/NADH just another substrate.
Ha. When I started typing, I thought I was going to help clear this up. Now it seems I just said what everyone else said. butlerw2:, give all that's been posted, is there still a question? Or has this all dissolved into semantics?