The word 'need' means different things to different people. The person trying to SELL you something clearly thinks you NEED a Sipario.
They COULD be right.
I've spend a couple of minutes with Google, and I note that the V3 expander is much more sophisticated than the korg module (whichever one it was) probably was. I'd guess it can do FAR more than you could easily control from the accordian as your 'controller'.
Even though the Sipario is designed for more complex set-ups than you're going for, it could still be helpful.
If, however, you do NOT want to get into more complex things, and plan to stick with the sort of things you were doing, then I'd guess that you do not NEED the Sipario. The extra bells and whisles (sp ?) could come later.
If you do get a Sipario, then you'll have 2 new high-tech devices to learn about!
Note that there's a very recent thread from Vincenzo regarding a Yamaha CP33 - he's using a Sipario. If he doesn't answer directly, you might ask him (via PM ?) about how he uses his device.
Geoff