Is cornstarch necessary in this recipe if you are using an icecream maker?

Yes. The cornstarch is there to absorb water. If you don't bind up the water, it will create shards of ice crystals in the final product. Jeni Britton Bauer's recipe was designed to create an ice cream that was supple and smooth. Each ingredient was added to create an ice cream that was neither too hard or too soft, with a luscious texture. I'm sure you can omit the cornstarch, but the texture would most likely be icy-er (sp?)