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The market for fingerprint readers for smart phones was almost nothing. Just look at the motorola atrix. Now Apple has added a fingerprint scanner and integrated it purely to make it slightly easier to log into your phone, and the iPhone 5s has sold out.The iPhone 5, 4s, 4 etc has sold out too, despite not having one. So I'm not sure it's the fingerprint scanner that's the differentiator here.
And the analogy is not apt for other reasons too. Apple didn't put a "fingerprint reader" because there was a market for it. They did it because it enables some things like easy mobile payments and better security.
Whereas with something like a wacom-like tablet capability, there IS a market, but it's tiny. And the technology doesn't offer much outside of that market (designers, painters and painting enthusiasts etc). There will be an improvement for handwritten notes and annotations, but that's not the major selling point of the iPad. If it was, Surface would have faired better too.
Still, they might add it at some point, if a dual-technology screen becomes cheaper and mature.
>The pen on a tablet is not a niche. The Surface Pro is a niche product.
Well, judging by unit sales, it very much is.