Having a reasonable suspicion that someone committed a crime is a valid reason for an officer to stop and search a person. However, that is not what stop and frisk policies are for. These policies give officers the right to detain you and search your person with no cause, a blatantly unconstitutional action.

Has this tactic reduced the crime level in NYC? I don't have data that shows causality, but I do have reports from the NYPD that show nearly nine out of 10 stopped-and-frisked New Yorkers have been completely innocent. 
http://www.nyclu.org/content/stop-and-frisk-data

So there's that.