Typical the reason for choosing one method over the other is economics.
In fiber optic fusion splicing a Fiber Optic Fusion Splicer machine is used to precisely align the two fiber ends then the glass ends are "fused" or "welded" together using some type of heat or electric arc. This produces a continuous connection between the fibers enabling very low loss light transmission. (Typical loss: 0.1 dB). Fusion splicing is lower ($0.50 - $1.50 each), the initial investment is much higher ($15,000 - $50,000 depending on the accuracy and features of the fusion splicer machine being purchased new or you can purchase a refurbished Fiber Optic Fusion Splicer from a reliable test equipment company for $3,000 - $10,000 based on model and features).
Mechanical splices are simply alignment devices, designed to
hold the two fiber ends in a precisely aligned position thus enabling light to pass from one fiber into the other. (Typical loss: 0.3 dB). Mechanical splicing has a low initial investment ($1,000 - $2,000) but costs more per splice ($12-$40 each).
Performance of each splicing method, the decision is often based on what industry you are working in. Fusion splicing produces lower loss and less back reflection than mechanical splicing because the resulting fusion splice points are almost seamless. Fusion splices are used primarily with single mode fiber where as Mechanical splices work with both single and multi mode fiber.
Many Telecommunications and CATV companies invest in fusion splicing for their long haul singlemode networks, but will still use mechanical splicing for shorter, local cable runs. Since analog video signals require minimal reflection for optimal performance, fusion splicing is preferred for this application as well. The LAN industry has the choice of either method, as signal loss and reflection are minor concerns for most LAN applications.