Using an expression pedal is a very different skill set for this application. To accurately recreate the exact level of harmony would be challenging, to say the least. To say more, the time to throw the pedal from min to max takes more energy, and timing than a simple press of a switch.
Korg my think that omitting an on/off feature was a good idea, but I can't imagine they did it on purpose. Every harmonizer I've ever had over the last 30+ years has had the option to tun on/off with a single switch. This expression option is a nice addition, but hardly a substitute.