<br />As mentioned, crimping action work hardens the connector barrel, making the joint mechanical strong and stable. If you solder a crimped joint, you may, in heating the connector, soften the copper, making the joint loosen. Now it is the solder only which provides the strength of the joint. But if the joint is subjected to vibration, the solder, in absorbing mechanical energy over a period of time, may crystallize, and the joint may actually fail altogether.<br /><br />Melted solder may flow through "wicking" action up the strands of wire, thus creating a stiff, solid section just behind a terminal. Since the terminal itself is made fast to a post or stud, the place where the solder stiffened section abruptly merges with the stranded, unsoldered wire is relatively unsupported. Vibrational flexing of the wire is concentrated at this interface, leading to hardening, embrittlement, and eventual failure.