I'm sorry but I would think that in a duo with just two instruments (melodeon and guitar you say) you would need to be extraordinarily accomplished to not need to vary that a bit with the addition of some vocals.

Where bands have (say) more members - more instruments - or more members who play several instruments and can therefore vary the sound, you might just manage it but even then, I would say the odd bit of vocalisation wouldn't go amiss. As has been said above, you don't need to be a perfect singer but many audience members can simply enjoy either following the story in a ballad, enjoying the wittiness of a skit of a more well-known song or simply joinging in a chorus. You're kind of leaving them out of it a bit if you don't let them join in and it might seem a bit, as has been said before, as if you are entertaining yourselves and not pushing to entertain them.

It's entirely up to you of course to decide what product you want to offer and then see how much demand there is for it. At the end of the day it is as simple as that really. Good luck.