Your points are valid if we're talking about a neutral salary negotiation, i.e., prospective employer asks, "What salary range are you seeking?" In this case,

Your points are valid if we're talking about a neutral salary negotiation, i.e., prospective employer asks, "What salary range are you seeking?" In this case, however, the disadvantage comes from a fixed number - your previous salary - not what you'd like. In a situation where you were underpaid, declaring your previous salary will definitely put you at a negotiating disadvantage.