Theoretically and often practically a mechanical joint introduces some resistance/reduction to the voltage flow. The older the joint, the more likely it is to create increased resistance to voltage flow, due to the deterioration of the electrical qualities of the mechanical joint.
Soldering wiring together can minimise these voltage losses and restore better conductivity or, reduce the chance of future losses of conductivity in newer wiring. It can also be a more strait forward process for installation.
Often stated disadvantages relate to wiring fatigue (soldered joint believed by most to cause fatigue in wire strands with vibration and the possible presence of corrosive residue from the solder process etc) and maintenance.
Usually, it's a choice between immediate/long term benefits and perceived future maintenence difficulties.
Properly crimped joints and the use of di-electric grease, will usually provide very good service. On the other hand if you have an older wire/joint, or a situation where once installed there is little likelihood of needing to separate that wiring, solder jointing the wires can be an excellent choice.