Arturia added the sequencer to the synthesiser; whereas, CA issued the synthesiser
The 1601 had some interesting features and quirks, so I would support a VM sequencer module. However, I wouldn't want to bloat out the 2600 - which is how I feel about the Arturia, btw (and I do own it, and much prefer the CA version).
I haven't looked closely enough at the 1601 to know whether or not a 1601 could be built from existing VM components (including the VM2500 1027 sequencer)