The advantage of an arranger keyboard is obvious--you don't have to sequence anything to perform with it. I agree that the computer sequence program would probably be the way to go if you need to overdub, etc.
The "pros" who look down on arrangers mostly do so out of ignorance. They don't understand the capabilities of arrangers. They think they are all like the first Cascio toys. I hope they remain ignorant because most of them can't compete for my jobs just playing sequences.
DonM