Auto-ISO onD300s

I was wondering if there any true advantages on setting the d300s to auto-ISO versus non auto-ISO. The reason i ask this question is that i had my camera set to auto at 200-3200 and was not happy with results. Most if not all of my images had a considerable amount of noise, even when shooing out doors on a bright sunny day. I readjusted using manual mode as need be and i was much happier with the results.

Is there any recommendation anybody out there can give me as to the advantages on shooting ISO in auto versus non -auto.

Any suggestions would be greatly appreciated.