The reason most pros don�t know about arrangers is that they are not marketed to pro musicians and studios by the manufactures, as they are not (And never have been) designed for professional use, but for the home hobby player who wants to play music with minimum effort and at a reasonable price. (A task to which they are entirely suited for)

If you look at the professional pro musicians then they always seek out all possible options that may give them an edge in song creation, (Some songs have even used a stylephone) and these musicians you will find do know about arrangers, and while they may not use them in anger or final production, (Other equipment being far more suitable) as a scratchpad they are a brilliant piece of kit, thus giving these musicians the edge over others.

Technology wise arrangers are antique compared to the professional equipment out there, but they are all that is required for the job they are designed for.

Bill