My thought process on this has been Nintendo can leverage the 50+ million 3DS users and 12+ million WiiU users into one unified platform, thus equaling ~60 million total console sales. This gives 3rd party devs a large number of platform holders, to entice them to bring 3rd party games back to Nintendo's NX.
Here's an example, using made up numbers to show my takeaway: Using Roger's picture of all the duplicate games Nintendo has had to make for two different platforms... let's say that over the life of the WiiU and 3DS they made 50 games for each. Of those 50, there were 40 games shared that were redundant (like smash and Mario Kart). By making a home console and handheld that can share a library, Nintendo has now increased their own output to 40 shared, plus 20 unique. Having those 40 shared now means they can devote devs to 40 *more* unique projects that wouldn't have happened if the platform wasn't shared. So, now Nintendo, by themselves can make 40+40+20=100 unique games for NX, instead of 40+10+10=60 unique games under the WiiU and 3DS. This added output would allow them to bring back neglected IP and create other new IP, like Splatoon. This in turn, makes their NX platform more appealing to gamers who maybe didn't pick up either a WiiU or 3DS before. It doesn't matter if people prefer a home or portable console, they all play the same software.
Looking at this from a 3rd party dev standpoint, having a home console platform that potentially reaches 60+ million gamers on NX means that they would be more willing to port over their games, versus just WiiU at 12 million units. 3rd party devs wouldn't care if you played COD on the NX home or portable, because those 60 million sales of NX hardware are *still* 60 million units out there. Plus, you will have some people (like myself) who enjoy being able to play one game both at home and on the go. I loved this ability while playing Monster Hunter 3 Ultimate. I could take my character on the road with me, and throughout the day get one or two fights in. Then go back home, sync over my save, and play on the big screen. Having more 3rd party software could very well entice more Sony or MS fans to actually buy an NX home console, which increases that platform's numbers. So now you may end up with something like 30 or 35 million home console sales, and 50 million portable. So, in theory, everything works together to actually *increase* hardware and software sales.