While it is true that 3D Touch could be implemented without the Taptic Engine, I don't think Apple would do that since haptic feedback is such an important part of 3D Touch. It allows you to sense when you have pushed hard enough to activate the function. In the case of force touch trackpads, it allows you a second level of pressure to preview a file instead of just select or open it. If Apple was able to make the entire iPad 3D Touch or force touch with haptic feedback, that would bring a lot more functions and would make the iPad a really great device since you can "feel" your content. The fact that it is missing on the iPad pro I feel is still acceptable for now since there aren't that many apps that use force touch yet. On iPads,we would need deeper uses for 3D Touch than just previewing. Maybe if it could be used to open contextual menus or even more options on an iPad that just wouldn't be possible to see on iPhones or smaller devices.