I solder and use shrink wrap whenever possible.
From experience, I find that wire joint repair over a period of time will corrode with exposure.
Whenever I have to troubleshoot a problem circuit, it's usually one that was hastily fastened with electrical tape, butt joint, or something stupid like maple syrup! When I remove the offending junction, the wiring has turned green from oxidized copper deposits.
Shorts can also result from wires that aren't protected with additional sheaths or grommets in places where there is a probablility of pinching.
Fires usually don't result from shorted wires arcing to exposed metal, but when the heat from such activity is enough to ignite the insulation, the surrounding material in the area or fluids, then you have a potential for a fire. Automotive wire is designed to withstand the heat from electrical shorts, but speaker wire is not, and I have seen some people use it as an alternative to supply power to such high amperage components such as fog lamps, car amps, etc!!
Fires can also result from damaged wire being replaced with one of lesser gauge and insulation. I recall once how a friend couldn't get his Bimmer started, and when I investigated, I discovered that he had spliced the aging ground lead with 20-gauge wire!!!
The condition of the insulation (which appeared to have suffered meltdown from the current that it was required to handle) made me realize that not being able to start the car probably saved him from an untimely engine inferno!