Your "inferior hand", as SkyDragon called it, is unable to punch. So you can only block with it, meaning you can't do double the damage with those two slots (as far as I'm aware with the current way 1.9 is going)
I was at Minecon 2013, and one thing I have to say is don't expect Mojang to say anything about 1.9.. (My Minecon cape is on an alt, by the way) There was a "Future of Minecraft" panel and all it was was the past of Minecraft and saying stuff like "The modding API will be added blah blah blah" Also, I saw krysyyjane9191 there and I only found out it was her around a month after the convention, when I saw her res xD
I really hope there comes a use for poisonous potatoes. They're so pointless, it makes me sad. It makes me wonder what they were thinking. "Let's add a potato variant, mmmmk, and not only is it not usable in any crafting recipes, but it hurts the player who eats it. And we should design it to look completely distinguishable from regular potatoes, so nobody can use it at all."