opportunities for abuse are always present in any law and by agents of the law under any circumstances....
by making the possibility that police will stray from proper enforcement guidelines is a valid reason not to have this particular law.....however, if that was the case, we could extend that argument and reneder all law valueless...
it appears you are presupposing or ascribing to the inane precept of adhering to the letter of the law (that notion supports infallibility of law and the human beings who enforce the law....it's logic reveals it to be little more than a wonderful political device to aid the criminal element)
and, I completely disagree with your statement on two counts:
I wonder if you'll change your tune the first time you're harassed without just cause?
1) you equate an innocent party being stopped to harassed....which is an unfair, yet, commonly used device to stir emotion
2) as far as changing one's tune.....personally, in light of the problem, if I was stopped, I'd be happy and thank the officer for doing his job because I have nothing to hide.