The crowded space is not because they feel other people will step in to help you, it is more of a disincentive from someone trying to rob you as there will be more witnesses later who can describe the thief and corroborate the victims story to police. While it is true some purse/phone snatcher type may attempt this anyway, they rely more on speed and not standing around conversing with someone for any length of time.

Anyway, this type of exchange might be best to take place at the police station, bank or somewhere with lots of cameras (and people). In any event I don't think getting into a car in a dark ally with stranger is wise under any circumstances, Bitcoin related or otherwise.