On the other hand, who else is offering a subscription service, which is basically what Amazon Prime is? Join for $79/year, get 12 books for free, video streaming, and free shipping.
Now, as a writer I have a choice: a second ebook format, or a new type of marketing. If I look at my epub sales and see a reasonably large percentage of my sales there, I'd turn thumbs-down on Amazon's subscription plan. If I see virtually nothing there, trying out a new marketing model for 90 days makes sense.
There isn't an automatic answer for every writer.