Providing clean needles to the drug-using population only serves to give drug users a way to continue dangerous, unhealthy habits. Providing clean needles to frequent drug users might help prevent the spreading of certain diseases, but does so at the expense of the individual using the drugs. Drug users are sometimes unpredictable and such programs may not even make a difference in the end. At the end of the day, there's nothing like catching a disease to act as a wake up call to stop using drugs.
For whatever reason when it comes to drugs, the words "harm reduction" are treated as obscene in my country. People are truly in denial about drugs. People are going to do them, no matter what laws you put into place. It's in our best social and financial interests to reduce the amount of harm people using drugs might do themselves. Needle exchanges accomplish this by reducing or eliminating communicable diseases such as HIV/AIDS, hepatitis and tuberculosis.