When you lift your hands off the keyboard your efficiency takes a hit
Yes, exactly! My earliest experiences with computers was DOS. Whenever I saw a Mac with a mouse, my thoughts were, what an inefficient system for interacting with a computer. Every time you lift your hands off the keyboard and have to roll that thing around on the desk, you lose time. When Windows came out, I used keyboard shortcuts whenever possible -- and in my opinion, Windows keyboard shortcuts are somewhat more efficient than the ones on OS X. I still think the mouse is not very efficient for writing tasks, and I'd rather do copy and paste with keyboard shortcuts than drag and drop, whenever possible. I don't think the mouse is inherently more efficient than touch at any task. People mention precise selection, but that can be solved by other means, such as zooming in, or entering numerical coordinates.
The benefits of a touch interface without depending on a mouse is that it is one less device you have to carry around. People keep saying that those who don't want a mouse don't have to use it, so adding mouse support doesn't harm anyone. But I worry that if Apple does add mouse support to iOS, developers would stop looking for solutions to making touch input more precise. Instead of making a system for zooming in when more precise selection is desired, they could throw in mouse support and call it a day.
In any case, for my work, which is translating short documents that are less than 10 pages, typing on the onscreen keyboard works fine. Especially with the 12.9 Pro, I can see enough of the text on screen even with the software keyboard up. This means I can get my work done whether or not I have an external keyboard with my iPad. It is extremely freeing to just pick up my iPad and go, without having to also pack a mouse and keyboard. That is what tablet computing is trying to achieve.
If mouse support can be added to iOS while somehow ensuring that no developer would ever write an app that REQUIRED use of a mouse, then I don't have an objection to adding mouse support. Just having an App Store policy saying apps would be rejected if mouse use isn't optional is insufficient in my mind, because policy enforcement isn't 100% fool proof. Come to think of it, if mouse support were added, developers would have to update their apps to work with the mouse, which could take time and resources away from other things they could be doing to enhance their apps. So no, it's not a case of an optional feature that doesn't harm anyone who doesn't want to use it. All those accessibility features that nobody uses other than the people with specific disabilities who benefit from them are costing Apple something, in money, time and resources. I'm grateful to Tim Cook for saying he doesn't consider ROI in adding those features, because if he did, they'd never be added. But adding mouse support, he should definitely consider ROI.