Didn't you say you had a passenger?
Maybe the cop was a total liar. Maybe there was some criminal report about someone fitting your description. I'm not willing to totally tie the hands of police officers just because you felt uncomfortable.
Once as a teen my friend and I were stopped by plain clothes officers while unlocking our bikes at a mall. It was a pain, it was embarassing, but I realized they were responding to a string of bike thefts in the area. It's called "life."
I agree, there was no harm done. It's just that if things are left open to abuse, someone will abuse it without question. They may feel justified in doing so in their opinion, but that doesn't make it right.