That's a very good point - This probably would have been a better laptop killer than Microsoft's surface Tablets, i.e. drop your Windows phone in a dock and you've a full Windows desktop-like PC, unplug it and you've everything with you. I'm sure they could have made a laptop-like equivalent of this, i.e. slot the phone in the side and the screen, keyboard and trackpad function a similar way, while the internal battery tops up the phone at the same time.
Although Samsung's DeX is a nice idea, I don't think it would be a big selling feature unless bundled with the phone. Anyone who currently wants Android on a TV or as a desktop-like PC can pick up an Android set-top box which comes with USB ports and a network port. While Samsung would have better performance, most games that depend on fast hardware will not work as they also depend on titling the phone and multi-touch to play them, which would be a problem with the phone placed in a dock. Other apps such as web browsing, video streaming and word processing, etc. all work just fine with most Android set top boxes.
I also wonder about the mobile operators who offer unlimited data plans. At present, most providers prohibit, cap or charge for tethering as streaming HD video on large screens is incredibly data heavy. As the phone itself would be doing the streaming for whatever screen it is plugged into, this could lead to just as heavy data usage as if the user was tethering its phone's data to a desktop PC. For example, a person living on their own could potentially use the phone's unlimited data plan as their main Internet connection, i.e. plug in the DeX for all their home web browsing, Netflix, e-mail, etc. A separate Tablet or PC would only work if unlimited tethering was permitted.