Banana shafts have a diagonal groove that crosses the oiling hole. It helps to oil better. They are not necessary, but are available. I figure if it were that big a deal, Mopar's aftermarket chrome shafts would be bananas...but they aren't.
Maybe MP doesn't always have the customers interest in mind in that if you gall up a shaft I'm sure they'll cover it under warrenty!!! They also use the Crane adjustable rockers which has had the problem with the adjuster tips wearing down. This problem has been out there quite some time and they still sell that defective POS but they don't need to change their supplier to fix the problem. My point is just because MP doesn't sell the shafts doesn't mean it's it's okay to run your rockers without the banna grooves.