The point isn't that I can't type adequately fast on the type keyboard, I can. However as in many things, once we get used to something better, it's hard to be satisfied with something sub-par again. If you are coming from the typical rubber-dome keyboard of today, then the type cover isn't too different. If you generally use a Thinkpad keyboard or a full mechanical one, then it leaves a lot to be desired.
Again, this is as much about how I feel using it long-term rather than how effective it is. A high dpi screen doesn't help me be more productive either, but I'd sure hate to go back from them.
I've thought about buying the portable Thinkpad keyboard, but at that point the entire device becomes as heavy as my Thinkpad, significantly less powerful, and more unwieldy. The point of making a better keyboard optional is that not everyone would want it, but there has been a significant movement back towards better keyboards recently. Microsoft should recognize this.
Wait a sec...I am coming from a ThinkPad X201 and I struggled with Lenovo to replace the Chicony KB that came with the machine to get an NMB fitted into it. This got the Lenovo design people in China and the country Managing Director involved in the case. They were very gracious about it. But that's not really the point - the point is that I am very familiar with ThinkPad keyboards and experience a very high level of comfort using them. I now use the SP3 and find that the Type Cover has a different experience, but not one that can be belittled. But then again, this is a personal observation and experience so it will, most likely, not apply to anyone else.