A cop can stop and search anyone he or she wants to based on reasonable suspicion, that is, a reasonable belief based on specific and articulable facts, that a person is about to commit, is commiting, or has committed a crime. Call it a hunch that you could explain to a reasonable person, if you will.
Giving police the power to search based on a hunch alone would be extremely dangerous. Since a hunch is not based on a reasonable belief, or on articulable facts, a hunch could be anything you wanted to be. In a perfect world a hunch would be justified, but in the real world, a justified, unarticulable hunch is no different from a race-based hunch or a prejudice-based hunch or simply an asshole-based hunch.
If they are held accountable to punishment when their hunch doesn't turn out the way they thought it would what is the danger?
I in no way call for cops to be immune or for them to have a free lunch when it comes to searches performed. Indeed they should be held accountable for the actions they take.
Cops have a job to do and it is silly to say hunches cannot play any role whatsoever. That is a hamstringing effort to stop them from succeeding in the task they are there to perform.
May be just semantics here, but this is more appropriately explained as 'stop and frisk.' Frisk is the least intrusive 'search' and the intent is to secure weapons to protect the officer's safety - it is not intended to arbitrarily seek evidence of a crime. It generally does not involve going into pockets or carried items that can be isolated from the person being stopped. Generically, this is a 'pat down' because that's basically all that is supposed to happen... those places where a weapon could be concealed are patted and if something the could be a weapon is felt - that particular spot is further investigated.