Yes, Hal, that does seem about right. However, an actual Performance consists of 4 Arps, not 5. For example, you can have Drums, Bass and Guitar arps playing for your left hand, and you can choose 1 sound for your right hand, like piano, for the melody. You can also use the right-hand sound as an arp, but then you won't have a sound to use as melody, unless you record the 4 arps first, and then play a melody over it. You also have 5 variations for every Performance, similar to the variations found on arrangers. Unfortunately, the Motif does not have Intros and Endings the way an arranger does, but you can certainly record them if you'd like.
An arranger is designed for you to just sit and play music immediately using Intros, variations, Endings, etc..... It makes it quick and easy to play any piece of music. With a Synth or Workstation, you could produce something similar, but it's a lot of work and time, hence the word "work"station.
A synth has features that go beyond an arranger's, which allow you to customize your music more to your liking, but in doing so, it makes it more complex and time consuming.
That's why I like the idea of having both.
One of the fun things you can do with a Motif is that you can change any of the 3 left-hand arps for any other arp. You have 7,881 arps onboard (the Montage has over 10,000!).
My wish is for keyboard manufacturers to combine both a workstation and an arranger's capabilities into one. Well, at least I can dream!
Thanks for your comments, Hal!