Do anti-depressants help most or all people with drug and alcohol addictions?

NO. No medication is always helpful. Given by experienced physicians antidepressants are often helpful in the appropriate setting.
Antidepressants. About 50%of drug and alcohol users also have a psychiatric problem, depression being the most common. Antidepressants will help reduce the symptoms of depression, reduce the need to self treat with drugs or alcohol, and reduce the chance of relapse to using drgs or alcohol. Yes, very helpful for many.
Careful. There are studies that have shown one agent, (zoloft) has led to increased relapse in alcoholics who began drinking heavily before the age of 25. It may have made them more impulsive.