To a certain extent, there is a definite advantage to a 'closed system' arranger. NOT a workstation (the more open and flexible they are, the better!), but an ARRANGER....
As anyone who has tried to convert a style from one arranger to another (even more recent arrangers from the same manufacturer!) understands, on anything other than the simplest, most bare-bones styles, the sound-set and the style are all of one piece. Without exactly the same sound, with the same velocity curve, same harmonic content, same sample velocity x-over points, same performance 'noises' (Mega-voice patches), it is a LOT of work to adjust even one style to approach how good it sounds on it's original arranger.
An 'open system' arranger only exacerbates those problems, with EVERYBODY that uses it having a different set of VSTi's, drumkits, piano sounds, etc., etc.. So basically, unless the 'open' arranger has a sound-set at least as advanced as the very TOTL 'closed' arranger (which they never do, due to the cost of development, and their attitude of 'but it's OPEN....", as if that is some magic mantra), the styles will NEVER be as focused and detailed as a 'closed' system, that the style developer has a mature and stable set of sounds to write the style on....
There are MANY aspects to an 'open system' keyboard that are of great benefit, but stability of sound-set to help style development is NOT one of them!
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An arranger is just a tool. What matters is what you build with it..!