there is many advantages
a single shock need not worry about having the same damping rate as the other one as it is lonely
almost always the single shock has adjustable damping as well a purpose designed rising rate linkage,which if done correctly greatly increases ride comfort,safety and handling
it is easier to design a swingarm with the proper direction of flex when the shock loads are not back near the axle
the rear frame can be designed to only need support the rider weight,as single shock designs are able to centralise shock loads away from the seat frame
the rear frame can be designed to be easily removable for service
etc etc
as far as the people i've seen convert to a mono it is obvious that most,they do it for looks and dont know fuckall about proper design
but most all of the customs we see here are all about looks not riding anyway
