Carbon credits make alternative forms of energy generation (wind, solar, tidal) price-competitive with polluting fossil fuel energy sources which are comparatively cheap. I.e. when the carbon tax is added to the cost of producing energy with fossil fuels it ain't so cheap after all.
If Australia had an effective carbon trading scheme our best and brightest wouldn't be taking their world-beating alternative energy technology to more enlightened countries like Germany and China (China has one of the largest wind farms in the world).
Nah, let's just bag it out and keep burning coal.