Let's go back to Apple's demo of 3D touch in terms of checking your mail. When you go into your Inbox currently, what do you see? You see who the mail message is from, when you received it, the Subject of the message, and the first two lines of the body of the message. You can go into Settings and change it to see up to 5 lines of the body of the message. But technically that IS you're preview of the mail message you got.
Now with 3D touch you press on that preview to see yet another bigger preview of your mail message, then press harder to actually open it up. Where as now it's a two step process of seeing the preview and tapping on it to open it, with 3D Touch it's now a 3 step process of two previews then seeing the actual message.
That's not efficiency, that's Apples illusion of efficiency wrapped around a shiny piece of candy and our eyes glaze over totally oblivious to the obvious that's it's the total opposite of simplicity and ease of use.
What we have now is simplistic and is all that is necessary to accomplish what we need to and see at a glance what we need to. If you need to see all that was shown in the bigger preview via 3D Touch, you might as well just open the message and read it.